Privacy often feels like a lost battle of the stubbornly distrustful to those who consider the perceived threats to be either outdated worries from a different age, or just laughable conspiracy theories. Thus the standard question is, why one would need privacy if they have nothing to hide? This is comparable to the following question: What is the point of an umbrella, if it never rains on me? The question draws the correct conclusion but the premise is probably wrong: It's quite unlikely that it never rains on you. So, let's get some education, let's find out, why we would want to hide something.
First up, I have to warn you, though: When you read this, I will know. In fact, I already know. I know how you found this article, which country you're from and roughly where in that country you are, how long it takes you to read this and whether you stop reading at some point, what kind of device you're reading this on and more. This interaction with me, me observing you when you actually believe there is no interaction since you're just reading a passive web page, like a newspaper, it's a bit scary at first. You can admit it, it's scary to have the newspaper stare back at you. But then you rationalize: "Ah, well. It's not that scary. I'm not doing anything I'm ashamed of. I'm just reading a damn article, after all. Let them watch! I don't care."
Conformation - Privacy from society
Shame and its overcoming actually play an important role in a persons development as well as the development of democratic societies as a whole. Where in ancient times the emperor or king dictated the norms and values of society, today these norms are developed by the societal discourse in the street as well as by the media, fueled by the impressions of recent events and whatever holds the public attention and ideally based on the rules defined by the human rights. In the past, the only way for society to evolve and alter these norms was a change of heart by the ruler. Or a revolution, that is. Consequently, normal people had to either hide their deviations from the norm as they had no influence on the course these norms would take, or stir up a revolution (which, as people were aware even back then, very rarely succeeds).
The prime example here is homosexuality. A long-existent phenomenon, homosexuality was frowned upon, even prohibited, for a long time. It was only until the advent of democratic societies and the attention to public figures and a wider community that more or less openly identified with homosexuality, that it gained wider acceptance in mainstream society. Privacy in this context protects individuals against (possibly misplaced) preoccupations, but on the other hand also hinders society to overcome these very same preoccupations by hiding the actual prevalence of a marginalized phenomenon.
Nowadays, it appears to be sporty not to be afraid of preoccupations, or at least not to show this fear to others, and to willfully provide a complete map of the self to anyone who even remotely cares. The underlying reason seems to be maturity: We are free from archaic conditions like shame. However, more often the reason is actually a yearning for validation, which a reveals a dependency that exemplifies the exact opposite of maturity: We are open with things that we know are accepted, in order to gain acceptance as a person.
Has this openness led to a more mature society today? I don't think so. In fact, I believe, openness without confidence has the exact opposite effect. It creates a society in which people are deeply dependent on the acceptance from others and even actively change their life to match those expectations: If we are open only with things that we know are accepted, but the expectation is to be open with everything, the things that are not so much accepted increasingly fade out of our life: we become compliant and social diversity crumbles. Maturity is the strength to do what you yourself want without being afraid of the reactions of others.
This is one reason why we need privacy. It is a right that you can exercise for all of your life, only partially, or not at all if you want to. This gives you the flexibility to develop aspects of yourself in private until you feel ready to face a wider "audience" with this new aspect of yourself.
Oppression - Privacy from the state
Dealing with societal pressure is one thing, and we might be able to learn how to handle it, but what about governmental pressure?
Already, our phones track us wherever we go and the police use this information to single out people in the vicinity of a crime scene. We use online services to stay in touch with friends, gather information and buy the products we need. What if the state could use all this data? In an authoritarian regime it would obviously be very easy to abuse the knowledge of this much data.
However, do western societies have to fear this technology as well? If anything, politicians assure us that we have to praise these technologies for their contribution to our safety and, especially with the ever increasing "threat of terror attacks", the question is raised, not whether what we do is right, but whether we do enough. Wouldn't it be even better for everyone's safety to actually prevent crimes from happening instead of merely bringing the perpetrators to justice after the fact? The technology is here, after all: Algorithms can learn to single out people that are likely to commit a crime in the same way as they are able to find out what you will likely enjoy next on YouTube. And consequently, German authorities already scan the phones of refugees to determine their threat level and the debate is on about electronic tags, too. Would you be willing to surrender like that? And even if you would, how would you know whether the profile created of you is correct?
Undoubtedly, we lose our freedom little by little by being the subject of surveillance for safety reasons. Is this acceptable? Let's take a step back: What is safety? Defining safety without using terms like freedom or integrity is very difficult. You will end up with a definition along the following lines: "Safety is the guarantee or feeling that nothing bad can or will happen." If safety is your main objective, who decides what exactly is bad? It's very subjective and potentially very easy for those in power to tweak to their liking without the people being able to change it. We already see the term treason being opened to broader and broader interpretations in cases of whistle blowers.
This is another reason why we need privacy: Giving up privacy to the state in exchange for safety is like playing with an open hand in a game of cards against the person who sets the rules. You will very likely lose.
Manipulation - Privacy from corporations
Digital technologies like computers and smartphones have had a great negative impact on our actual privacy, while letting us believe we're still alone when we read our mails, the news, or watch that funny video. We think this way, because we're used to technology that works this way: passive tools that we obtain once, that we are owners of and that we can use to our advantage, be it a screwdriver, a newspaper, or an oven.
The internet is different, though. It's interactive. Every time you click, the click can leave your computer; every time you send a message to a friend, the message leaves your computer, your phone. They travel half-way around the earth, being translated back and forth from and into various "computer languages", passing through computers until they reach their destination. All the computers in between can deduce a great deal of information from the data they watch pass through: Your network provider, for example, knows which websites you visit and when. The websites you visit know what you've visited before, where you are located and more. But of course the most interesting data is the information you give up willingly.
The up-side of entering information is that you get something in return: tailor-made services and offers. We are recommended videos that we actually like, restaurants that are actually near us, products that we actually need, friends that are actually compatible to us. All those apps feel like good friends that know us, our desires and preferences, and make suggestions that we will like, often free of charge.
Is there a catch? Well, the obvious problem is, as a wise man once said: "If it's free, you're the product." Many people seem to know this, but few actually realize what this means. If you get a calculator and dabble a little with the numbers, you'll find that you'd have to pay less than a cup of coffee for a year of usage of Facebook's services. It's really that cheap. Why don't they make you pay that amount and be done with it, though? Firstly, it's clear that more people will try and get hooked on something that's cheap let alone free. But even more importantly, they simply don't need to charge you: You're way more valuable than the two bucks it takes to run the servers that you so willingly tell the story of your life to.
"So, they take my data in exchange, big deal", you say. Let's embark on a little endeavor into market theory: In market theory there's consumers who have the freedom of choice to choose the producer they like, and on the other hand there's producers who compete for consumers and have the need to sell products. When we use social media and other apps we feel like we're the consumers, when in fact, it's the other way around. We have the need to sell our data, because we want to benefit from the services' offers. The services have the freedom of choice, which demographic they would like to entice. Facebook and other social media sites are comparable to traditional product testing and market research organizations, in this model. They provide consumers (other companies) with information about products (people) they might indulge. Of course the tables are not exactly turned as companies always try to get as many people as possible to buy their stuff, while consumers normally don't buy as much as they can, because they can't afford it. Today's services can afford this, because the simple upkeep of the technology behind the services is so cheap.
There are two things that are worrisome about this: Firstly, while you might profit from this at the moment (due to tailor-made services), there's also companies that you probably wouldn't want to know that much about yourself, while having no control over who gets a hold of your data. Imagine your health insurance company knowing how unhealthy your nutrition actually is -- they surely might want to update your plan accordingly. Imagine your employer knowing what you buy, your political stances, or how long you were actually up last night -- they probably might want to reconsider your next raise or even your promotion.
The second aspect is even more important: Do you know why companies pay so much for ads, when people just ignore them? Because they work, even if you try to ignore them. When you as a consumer buy something at a store that doesn't suit you, you give it back. When a company as a "people consumer" faces a demographic that doesn't engage in their stuff, they work it. Hard. Until it does. As I noted before, almost all services nowadays are not one-way streets anymore, they are interactive. And as they want to entice as many people as possible, they not only carefully observe what you put in, but they can also control what you get back. They can carefully construct input to nudge your brain into a specific direction. People trust what Facebook shows them in their timeline, but nobody really has a clue what does and doesn't end up there and how prominently it is displayed. Researchers have verified that the order of the entries in people's Facebook timeline could be utilized to control their mood.
This is another reason, why we need privacy: To keep our integrity and identity from being manipulated by corporations.
Degradation - Privacy from political actors
We've seen that corporations can influence what you buy, turning you into a product. But there's another market that works very similar to the retail market: the market of democratic opinions. It will not surprise you to hear that in the same way that corporations can influence you to buy something, they can also influence your political actions. Scientists have already confirmed the possibility of influencing a person's decision on whether to vote and the rise of fake news should have made clear that what you perceive to be real shapes your stances immensely, which means our political actions are not safe from manipulation.
In the end, the personalization of (news) services robs us of the societal synthesis of reality. We construct reality by having it constantly clash with the reality of others: Discourse is a way of consolidating realities. The basis of such discourse is a common ground of facts that we agree on and try to draw conclusions from. But as we transition to more and more personalized sources of information, political discourse becomes absurd, because we don't have the same information, the same picture of the world. We talk at cross-purposes. This is already apparent today, when right-wing and left-wing supporters talk to each other. Either one tries to convince the other of being victim of an ideology. The source of this chasm is simply different input, that describes reality in a way that makes certain explanations applicable and thus shapes our stances.
This is the most important reason for privacy: It ensures the capability of the democratic system to supply people with as objective information as possible, sustaining political discourse on a common ground of facts, without the temptation to tell people what they want to hear or what they likely will click on.
As we've seen privacy is basically about trust. Trust can be healthy in interpersonal relationships, but in most cases trust is mutual. You will probably only trust someone you know well and if you trust them, you can trust them with knowledge about you, the real you.
On Facebook for example, you may contain the most personal things within the circle of your close friends, but in order for your friends to know them, Facebook has to know them, too. So, in our scenario Facebook must be like a close friend to you. But how much do you actually know about Facebook? About it's values? It's goals?
Contrary to intuition, privacy is not about emotions, it's about reason: Taking trust out of an otherwise unbalanced equation. If you are looking for an objective information or service, be it news stories, good restaurants, exchanging stuff with your friends, or, in case of the state, freedom and security, the other party shouldn't require to know who you are. Just like democratic elections are by definition anonymous: You may trust the person collecting the votes, but you shouldn't need to in the first place.