Quite a bit is the networking. Sometimes you get easier access to certain publications. Horror Writers of America has member only anthologies for example. Membership makes you eligible to vote on awards. It can also get you a certain cache - becoming an author member of RWA or an active SFWA member means that in theory you've sold enough in the genre to make you a 'professional.' RWA used to have health benefits but seems to have discontinued that in recent years. Basically, they're supposed to be professional organizations as opposed to social ones.
Re: the whole groping thing. Oh yeah, one thing about the organization is that fun never stops. The survivors will no doubt be feuding amongst themselves when they're down to the last 2 members. :-(