Apparently, everyone knows that you need to cover your license plate when you put a picture of your car in a Craigslist ad. Except... you don't. Nothing changed legally from when no one used to do this. This article here gets at it a little, suggesting that people can find out personal information about you, such as your address, using your license plate number.
But it’s really a deeper confusion than that. What’s actually happening is that mentally, people are treating their license plate like their Social Security Number. The reason we protect our social security number isn’t to keep our information private, because no one cares about keeping their information private. Social security numbers are dangerous because of identity theft, i.e., they can be used to set up bank accounts, acquire credit cards, make purchases, and default on debt. That is bad because it ruins your credit.
You can’t set up a bank account with a person’s license plate number, end of story. But somebody thought you could and "protected himself" first, the next guy copied him, now we all do it, just to be safe. The whole thing is stupid.
But the evolutionary source of the behavior isn’t stupid, it’s marvelously adaptive. Thinking through every source of danger and applying reason to discovering the proper protection is costly in the natural selection sense. Time spent wondering about which personal numbers need to protect is time better spent looking for food, acquiring clothes, and providing them to your children.
Instead, natural selection has produced humans who know one little trick: just copy the precautions taken by the next guy. We think we fasten our safety belt because we’re smart, but we actually do it because everyone else does.
Everyone knows that to enforce certain property rights, you should mark the boundaries of your land with signs that say “Posted No Trespassing.” Right?
Except... What is the word "posted" doing there? This article is too kind in suggesting that the “Posted” is for emphasis. "Posted" is there because, once upon a time, someone read a legal opinion that said something like "...because the property was posted no trespassing, the owner had the right to..." In the case, "posted" was a verb describing signs which said "No Trespassing," but the next guy came along and thought it was one of the magic words. So his signs said "Posted No Trespassing."
Then we all copied him... just to be safe!
For like a hundred years, and counting...