The Hypothetical propostition
Hypothetical is like a pub. You get yourself a table, all your friends join you and you just chat about stuff, share anecdotes, that sort of thing.
Most community sites are subject based. They bring together groups of people interested in rockets or watering cans from all over the world. They allow people to discuss their interests with people they could never normally meet. But on those community sites a core group always builds up and they sometimes want to talk about things other than the specified topic.
Meanwhile, there are many many groups of friends in the world who've been separated for some reason. It happened to me when I moved away from my university town, or it could just be that you're all at desk jobs in different companies and you want to keep in touch.
There are lots of little groups that exist in one way or another who don't have somewhere online to just hang out.
Hypothetical is a community site that gives established groups a nice home. You can keep in touch with the group as a whole from any location. You can share files. You can do lots of things.
Tools like email and instant messaging let you communicate with individuals very effectively, but with Hypothetical you can tap in to the magic of the group as a whole. Hypothetical is non-realtime, which smoothes out the difficulties of internet communication - timezones, access to machines and so on.