I have to admit I ask myself this all the time.
The internet is amazingly empty of good reasons why a show should or shouldn't charge an admission fee. Here are a few things I found, and a few hypothesis of my own.
1) To create the illusion of prestige. Yep. Seriously. To make you think that whatever is inside is so good and so exclusive that you need to pay for the privilege to spend more money once inside. There is an area of thought that believes that the physical act of paying an entrance fee changes our perception of the goods inside so much that we are willing to pay a higher price for them than if they were exhibited at a free event.
2) To raise money for charity. Often the entrance fee goes all or in part to a local charity. Sometimes the entrance fee is lowered or eliminated with a donation (like a canned good) that will go to a local food bank. Always check and see if this is 1) a cause you ultimately support and 2) what percentage of the fee is actually making it to the charity. Because it is hard for me to trust that money actually makes it where it is supposed to, I always bring things (like clothes or canned goods) when I have the opportunity. I believe they are much more likely to make it to where they are supposed to in the end.
3) To support the artists. These shows are expensive to do. Insurance is astronomical, rent for buildings or parks is skyrocketing, advertising costs more and more, infrastructure (like garbage cans & lights) must be paid for, and musicians/entertainment also get their fair share. Rather than have high fees for the artists to attend, an entrance fee is used to make up the difference between what they pay, and what is needed to provide a good, safe, and fun environment for the show to take place in.
4) To make money. There always will be those places that cover the aforementioned costs with a high artisan fee. They make sure they break even before the show even begins. The entrance fee is their income, their pay for all the hard work they have put into the event. If they work hard and draw a big crowd, then they get a great payday. If they work hard and draw a big crowd, most likely the artisans will have a big payday as well, sales should reflect the gate attendance.
Often these reasons are intertwined. It can be difficult to decide if you want to pay and entrance fee into a show or not. Check the events web site for coupons, ask your friends if they have gone before, check with artisans you know who are exhibiting there and look for candid advice.
I also have to ask myself if I should participate in shows that charge a
fee, at all, some of the time, or never. That is a question for another