They are just for fun. We're hoping to have some group based competitions soon where groups can compete against eachother. Each group also has it's own private forum for group discussions, plans to compete with other groups, etc.
Some groups are more restrictive on who can join but there are some that will let anybody join. Just look around in the Group Talk forum under Sub Talk for details on each.
You don't have to join one though. I'm neutral and loving it.