If you were the lead on a project for a company and a vicious competitor happened to find out that you were a recovering alcoholic, what would happen if they waited until the wife and kids were out of town for the weekend and they left a wine sampler on your doorstep?
What if your neighbor secretly hated you for something they never bothered to tell you so they waited until you were on vacation and posted a prank ad in the paper that everything in your house was free for the taking.
You might be amazed at how much trouble you can cause for someone if you know enough about them. Think about the movies where a group of people face impossible odds to rescue hostages, rob a bank, or other fantastic feat. The only way they're able to pull it off is because they already know all the security, the layout of the buildings, the defensive measures etc.
Just like in the movies, the more someone knows about you, the easier it is for them to manipulate or destroy you. Don't let that happen! Learn that your information is important and it should be protected.