What is the point of buying college books when professors give you notes on the overhead?

Because unless your teacher is awful, the books give you more information than you get from just the lectures. Likewise, the lectures and discussions should give you more information than what is in the books and should also clear up any misunderstandings or questions students have about what they read.What is the point of buying college books when professors give you notes on the overhead?
Sometimes they cover everything in the book in class (which can be frustrating!), other times professors will just assign a book, and expect you to read it because material from it will be on the test. First tests are always an indicator of what does and doesn't work (i.e. if the tests are all from the lecture or from other sources).What is the point of buying college books when professors give you notes on the overhead?
You are supposed to read your books. The professors don't assign "reading assignments". They just expect you to read the book.What is the point of buying college books when professors give you notes on the overhead?
They may test you on things that are in the book but not covered in lectures. Or they may assign homework problems from the book.