A lot of the time such "friends" are, say, people who went to the same school or something and aren't really friends in any traditional sense. But perhaps one day there might be some reunion organised, or something. Also it's just hard to say no to a friend request -- feels a bit rude. So you end up with a bunch of people you rarely speak to who can see your page, filled perhaps with stuff they don't care about.
But then there are some people you would like to keep in touch with regularly, and it turns out that Facebook's a useful way of doing that. Not the only way, perhaps, but a handy way all the same.
And then you don't have to update it with such random junk. Some people do, and that's fine, just ignore it. Doesn't detract from the rest.