Oh, you didn't know about that?
Everybody does it these days.
Here's how it works: I provide some kind of useful service for free and and then I collect a whole bunch of information about your behavior. But I need money so I run ads on my free service. To get more money, I want to make sure the right people see the right ads, so I use the information I've collected about you to pick just the right ads for you. So I'm not really selling your data, just using it to make money.
(Just to be clear, I'm not actually doing any of this because I'm not collecting any data or running any ads. But companies really do this, just so-ya-know!)