The more someone knows about you, the more it can be used to manipulate you for *their* purposes, which might not be to your benefit.

• For example, sales, fraud, political manipulation, ID theft, social engineering (to further compromise systems), or...
• ...or forming your opinion to their preference, vs. your checking and considering carefully the multiple sources that *you* trust.
• Or rent/sell a house while the owner is on vacation.
• Or learn about your habits or vacation plans from social media, so they know when to rob you.
• Sometimes people even commit crime in others names, because they know enough to impersonate them while committing the crime.
• Or get loans in your name, then you are harassed to pay them back.