Companionship's a part of it. Also, I suppose if an event benefits a charity, there's that.

Were those self-supported rides that you saw called "brevets" ? Those are unsupported (or minimally supported) endurance rides, although they tend to be longer than a century. We've been talking about brevets here a lot lately and many have posted about their brevet adventures recently, especially the uber-long distance crazy women like maillotpois and Fredwina. (I consider myself merely a crazy woman in training).

I can't say I've actually heard of any wholly self-supported organized century rides. There's one in my area called the Mt. Hamilton Challenge (which was held last weekend) where riders are supposed to bring their own food, and place it into bags to be delivered to various rest stops where the riders can get at it, but even so the MHC has those rest stops manned with volunteers, and roving SAG support as well (I think)