To stop them from seeing what they get up to in their personal life, I imagine. I know I'd consider doing it, had I any family other than my cousin and my sister on facebook. A week or two back my aunt actually added me - but she's since deactivated her account so it's alright, haha.
One thing I don't really get is deleting people you don't often talk to. I mean... it just seems pretty unfriendly and standoffish to remove someone just because you don't talk to them much or at all when they do absolutely no harm being there. It's almost like there's some standard you want your facebook friends to live up to which seems way off to me. I just add anyone I've had a nice chat, a good night, whatever, with so I can keep in contact with them if I want to and if not then next time I see them do anything I'll at least remember them for anything we did together. Removing people for the sake of it just makes no sense to me, honestly.