I've been to a handful of art fairs so far, and here are a few insights I've learned from the experience. Art fairs can be expensive. There are application fees and reservation fees, and then you have to consider meals, incidentals, and any travel that may be involved. You may find a need to invest in new marketing materials or inventory for potential customers.
Preparation is key. Flights of stairs, electrical outlets, the weather -- all these things can affect what you choose to bring and how you choose to set things up. Your best bet is to bring more tools than you think you need, and a few extra snacks. And to be on the up & up get those tax licenses and permits taken care of in advance.
Your milage may vary. No two art fairs are the same, even in the same town or local area. And over the course of a multi-day festival, every day of the same festival may be wildly different. It's worth trying a few different fairs so that you can get a better sense of where your audience is and what resonates with them. Keep track of what works and what doesn't; it's good fodder for planning your next fair.
In the end, it's mostly marketing. An art fair is just one other way to get your work in front of an audience. But the best part about this kind of marketing is that you get to interact with your audience. By far my favorite part of the art fair experience has been getting better at nailing my elevator-speech about the cyanotype process. Connecting with people and being able to share what I'm all about is one of the best parts of making art, and one of the reasons why I keep doing it.
P.S. Joanna Gresik has a great blog post about things you should bring to an art fair, as well as a great booth setup to look at!