It's a proven fact that there are more strangers than people you know. While there may be some percentage of complete strangers who will treat your private life with the same care and diligence as a close friend or family member would, odds are that most won't. Though most people aren't dangerous, some are and they don't come with forehead labels so you can tell the difference.
Why should I care? I've done nothing wrong
According to who? Some would say that because you have a house and a TV and maybe a nice phone, your privilege grants them the right to target you for burglary. When I was a government worker, that fact alone justified harm in some people's minds (stick it to "the man"!) while elsewhere in the world, simply being a US citizen means you're guilty and deserving of death. What if you simply look similar to a known terrorist? Did you leave a big enough tip at the restaurant?
(from Reddit): mom's ex boyfriend. He was a waiter at a very elite restaurant and had normally expected very nice tips. Some guy left him a $5 tip for an over $100 bill and he got pissed and posted the customer's information on Facebook.
The world is made up mostly of people who don't know or care about you, but might depending on what information they find about you. Whether you post it yourself or its exposed accidentally, all it takes is one errant tweet or photo taken out of context to get you fired, harassed, or sent death threats.
Outrage doesn't stop to consider or wait for an explanation. Evil doesn't feel remorse or mercy. Once someone decides they don't like you, the ways they can harm you is limited only by what information they have about you and their imagination.
Did you know that thieves are watching social posts to find out which houses are unguarded while the family is on vacation (and if your insurance finds out, they may not pay)? What if someone doesn't like how you treated them and can find out where you work? It's not especially difficult to contact someone's boss and make a case for why you shouldn't have a job anymore… and depending on what I know about you and share with the boss, it might not be that hard of a pitch.
It's worse than you think
Snooping and judging is the new norm. Do you imagine I won't dig up everything I can find on someone who wants to date my daughters? Do you still think you can get a job and not have your own social posts brought up in the interview?
Our courtrooms live and breath on the evidence that comes from your online activity. Lawyers, co-workers, or ex-lovers may all be motivated to paint you as someone you're not. Could they use your emails and comments to make you look biased, predisposed, violent, or whatever else they need to win? Sometimes police and politicians are under so much pressure to make someone pay that they're not very careful about who actually goes to jail. It's in your best interests not to hand them the knife they stab you with
And then there's the everyday data gathering businesses and online sites do every day to profile you and exploit your weaknesses for money. Even when it's as simple as giving private information to the dentist or rental car company, information they hold has a habit of leaking away to even more people you don't know. When you give information to organizations like these, you have no idea how many people or what kind of people will end up in possession of it leading to ID Theft or worse.
Privacy is simple risk management: there are far more people in the world that you don't know and trust compared to people you do. Giving information to people who's motivations and capabilities are unclear is not smart and not safe. Stop saying "I have nothing to hide" and start saying "why do you want to know?"
Until and unless you can determine a specific and valid reason to give up private information, you shouldn't give it. Learn more about protecting your data and identity in my Goodbye Identity Theft course.