I've noticed specifically with undergraduate physics courses that the textbook will usually get bundled with the online homework service that the school is using, and that part you're required to purchase if you want to do your homework.

Nowadays though, if a professor states that a textbook is required and there's no online service involved, 80% of the time the professor is exaggerating and probably hasn't even read the book themselves. The other 20% of the time, students will generally just find the pdf for free online somewhere (As a CS student, I tend to find my books on GitHub).

I know I'm digressing from what you asked a bit here, but I just really need to take a moment to highlight that the textbooks that are "required" today are not nearly as good as the textbooks you likely went to school with (I'm making a bit of an assumption about the era of your schooling here, so correct me if you only just recently graduated). There are way to many instances of some no-name authors getting the shot at publishing with O'Reilly or Pearson. The content will be mostly correct, but they're never truly illuminating.

On that note, the only textbook I still have is a 1980s Physics 100 book, and I still pull it out occasionally for reference.