I don't "understand" it either, but I'd imagine their line of thinking is that you've been using facebook for years, liking things you like, going to movies/shows you like etc that won't end up on a form you fill out. People want to be done with the form filling portion very quickly and won't put in very much detail. When scraping your facebook data, they get a very large amount of personal data and they figure they have a better chance of linking people together who actually do have several common interests, instead of: "I like music, and movies and the outdoors"