To answer the question about why to join facebook, it's because people like to be social. I am acquaintances with a ton of old high school friends, and a ton of friends I've met at various different workplaces, and while we don't talk on the phone or see each other frequently, if ever, I still like to see pictures of their babies they post for instance, or just see what types of things they randomly post about. The same goes for many members of my family whom I don't see frequently.
If I enjoyed a movie, book, video game, whatever, maybe I'll post about it on my wall and see which of my friends have something to say about the same subject. Facebook is social media, and that pretty much explains everything there is to say about it. My friends and I also use it constantly to create events and invite them to them. Things like a house party become infinitely easier to plan when you can just throw up a facebook event with the address, time, and other details on it, and people can RSVP without having to call 30 people.
If you're not a particularly social person, or don't care to keep up with people who don't qualify as close friends or family or whatever, then I can understand why you wouldn't have a desire to use it.