kinda like the informal CCCC clan or the CCSC clan or the CCJC?
but also being part of a "group" the admins and mods of the site can create closed off sections of a forum for just those people belonging to those groups.
it is good the way it is here though having almost everything open, some forums use them and it causes drama among members. Kinda reminds me of the Dr. Seus book about those things(i think they were called sneetches) with the stars and those without.
I use them where I work we use phpbb which is very similar, which is a good example to use group memberships. We have projects where we have "participants" of a study and "evaluators" and you don't want the participants to see what the evaluators are writing or else it would ruin the study or project getting "tainted" data collection.