David Friedman, on the newsgroup rec.arts.sf.fandom, explains something I've struggled to put into words about why I go to science fiction/fantasy conventions.
I think part of what makes fandom and conventions work is the nonpecuniary gain from trade.
If you are a moderately successful sf writer, from the standpoint of your neighbors you are nobody of any special importance. You may make a reasonable living, but you aren't rich and you aren't famous.
From the standpoint of your readers, however, you are a star. Most people (not all) like being stars. A convention puts the author together with his readers. They get the fun of interacting with someone they think of as famous and important, he gets the fun of being treated, for a weekend, as famous and important. (quoted with permission)
Hanging out with (some) authors is a lot of fun: they are intelligent, good conversationalists, and funny. Often they have information I don't have, and they enjoy sharing it. They're mostly all storytellers, and I try to be a good listener.
Another reason I go to cons is to spend some time with (one of) my tribe(s). Fans are my tribe in a lot of ways, and a convention is one of very few places where I don't have to either constantly explain what I'm talking about or receive rude remarks and looks--ridiculing sf/f fans is still socially acceptable, much like making fun of fat people.
At a con I can fully express some of my characteristics and enjoy my pleasures without fear of being outcast, of being too different. In fandom I am "one of us," not one of them. It's nice to finally belong somewhere after all the years I spent trying to belong to all the different tribes I still feel part of, even if other members don't acknowledge me much.