They have a purpose, and that purpose is to maker things easier on the admin, for example, if you have a section for members above a certain age, currently you'd have to manually add that usergroup to their account, and ONLY admins could do such, with joinable groups any group leader can add them, making it easier on admins.
I think you're also thinking of social groups... because of course joinable groups would have such a setting.
With all the +1s in this thread I think it's pretty clear. ;)
Once again, it's not social groups, it's joinable user groups, the idea isn't like clubs at all and segregation isn't at all a part of it, it's about allowing users an easier way to be put in to certain groups, given access to certain forums, and given certain permissions. It's not social networking at all, and it's a feature on almost all the other major forum software already.