Answer: They're fun! And you can have fangeek moments and no one gives you strange looks. And you can network with other people who have similar interests.
I spent last weekend in Phoenix AZ at the First North American Discworld Convention. Sir Terry Pratchett is amazingly funny, friendly, and a true gentleman. The con was well-run and very enjoyable.
And I agree with Sir Terry's comment: "I've never seen so many corsets in one place at one time in my life!"
Thanks to the committee and the guest of honor and the others who attended for making it a great experience.