As a middle school librarian, I run a book fair twice a year. Typically the fair takes about a week of my time. My school and school district both support the idea that as a librarian I have 2 or 3 book fairs a year. If they didn't, I wouldn't have them. They are a lot of work. I can't emphasize that enough, a lot of work. When I was younger, the work didn't seem as impossible as it does today.
But, thinking past the work and the required nature of selling things at school during the school day, I can see some benefits for the book fair. It obviously provides books the company donates to the school and the books students buy to enjoy. At a book fair kids learn to weigh their wants and prioritize. They see that some people have a lot more money to blow than they do. They learn that just because it looks pretty or inviting doesn't mean it isn't junk. They learn to figure in the tax. They learn that while $20 seems like a lot of money, it doesn't buy much at the book fair.
I have finished my last book fair for the school year. It feels like a relief to have it packed up and done. The book fair is a learning experience and it is a little fun once the work is done.