This question has been on my mind a lot lately.  Especially considering that I have chosen to take this year off from conventions, with the exception of Illuxcon in September.  After all the money, blood, sweat, tears, and coffee, are cons worth your while to sell at?  Here is what I have learned after 10+ years of doing conventions as a hobbyist and 4 years as a professional artist:

The Pros

This is the number one reason anyone thinks to attend conventions not as a fan, but as an artist.  You get valuable face time with people who might like your art and start getting your name out there on the tongues of people, which is an especially good move if you are the kind of artist who plans to make their income selling art directly to their fanbase.

Face to face selling is also far more effective since your fans can get to know you as a person so they have more of a reason to buy your art. Sounds weird, but having a personal connection to a REAL living person can be very powerful!  Meeting someone in person allows us to want to emotionally support them even more than if they were a faceless artist online whose art we merely consume without consideration for the human behind them.

2.  Marketing Yourself

The other main reason we as artists choose to attend cons is to meet with the folks that can put us in touch with jobs.  Art directors, game developers, publishers, etc.  You'll probably never meet these awesome folks who lead you to professional opportunities unless you go to conventions!  The downside, these folks may not be at smaller cons so you'll have to attend the larger ones which may not be local to you. True, you can still email in a portfolio, but I consider face to face interactions to be more memorable/powerful...