Some people need to see information to be able to remember it while others need only to hear it. Most combine these two and have to do it as well. Textbooks provide students with all three. We hear the information from the teacher's mouth, while also having the book to visually reinforce it. The practice problems and questions in the book allow us to do it.
Textbooks are both the slackers' and the smart students' dream.
In classes where I can't stay awake or can't understand the teacher anyway, the textbook is a godsend. In other classes, I'm way beyond what the class is covering, so I just read ahead. Or I skip to a more interesting chapter we probably won't get to. If we had ideal teachers, we wouldn't need books. The teachers would tell us everything we need to know while being creative enough to keep our interest.