Facebook Groups

Can anyone tell me what the point of a facebook group? I am member of a few facebook groups but it appears to me there is absolutely no point. Things are never updated on the groups - nobody writes on walls. I am in a group from my elementary school, my grad class. I am in two groups in memory of old friends that have passed. I am in a group of band of guy I went to high school with. I have unjoined this group twice now but he keeps sending me new invitations so I fell compelled and I finally joined a group about gas prices after 11 invites to it. I check these groups occasionally to see if anything is happening in them but to no avail there is nothing.