The professor might benefit financially from the sale of the book, e.g. if they're the author. It's quite common, really.

Yeah, I had to buy one or two really shitty homemade-looking books the prof wrote. On the other side, I had professors that wrote their own curriculum and did their best to enjoy we didn't waste money on unnecessary books.

My experience was different. I bought them in college whenever possible and none of the professors cared. Often they were exactly the same content, although sometimes the questions differed. Probably varies by institution.