Publishers come out with a "new edition" that changes the homework problems around or rewrites them so that when the professor assigns chapter 3 problems 1, 6, 9, 30, 67, 83 if you have the wrong edition you can't do the homework. There are ways around buying the books, and so far I've been lucky with either finding torrents, international editions for texts I had to have, or other methods. I've bought a handful of books for my courses, but mostly I end up spending money on the online access codes that I cannot get around purchasing. It's a huge scam but whatever. Some degrees are worth the cost and effort. Most aren't.