A con is an interesting mix of social and business activities. For some folks, it's work. There are published authors looking for readers, agents meeting clients and book editors, magazine editors trying to drum up some business, and booksellers and other retailers hoping to move some stock. For other folks, it's purely social. The eat and drink, they talk, they go to panels and readings, they shop in the dealer's room, and they basically just relax.
And then there's the overlap. A lot of the folks you'll see camped out in the bar are actually from the first group, the pros who are working. For writers in particular, cons are a chance to get out of the house and talk to flesh and blood people instead of just the ones in their heads. Writers who are introverts will still go to cons, but they tend to keep their heads down. It's the extroverts who party until all hours and talk nonstop for four days.
Where else will they find such a collection of interesting, well read folks who all love the same kind of reading matter? Somehow it all comes together!