Trust me, I get it. Textbooks are expensive, they're boring and most of all, super time consuming to read. And while you may know someone or believe that you yourself can pass a class without cracking your textbook open once during the semester, sorry to say this, but you're wrong.
1. Most of the time, professors build upon the content that is in your textbook
You will typically be assigned a reading prior to your class. Now there is a good reason that the reading was assigned before your class meets. Readings are assigned this way so that you have some basic knowledge going into your lecture. Textbooks and professors work hand-in-hand to help you understand the material. Professors teach the material in the textbook, hopefully clarifying anything you could not quite grasp on your own. So in order to make the most of your class time, you should probably read your textbook.
2. Sometimes, the textbook is better at explaining concepts than your professor
I don't know about you, but I have had professors who have talked in circles around a concept but never quite actually got to the point. If you have had a professor like this, the best place to go to try and understand what you're learning is the textbook. Generally, textbooks are made so that they can be understood by most and they are usually very straightforward.
3. The content of your test may come from the textbook
If you read the syllabus for your classes, most of them will say that tests can pull from material from both the textbook and lectures. Because the amount of time professors have to teach is so limited, they are simply not able to teach you everything that you need to know for tests. There are instances when the professor has never once even mentioned a concept, and yet there it is, staring at you on your test. If you don't want to find yourself in this position, read your textbook.
4. Your note taking abilities will improve immensely
If you are someone who has never read textbooks or learned how to efficiently take notes in high school, ahem me, then you will have a very hard time succeeding in college. Everyone should be taking notes in class because your notes are your lifeline to an A. But if you are not that sure as to how to take good notes, taking notes while reading your textbook is great practice.
I personally find that the most difficult thing about note taking is keeping up with the lecture. But while taking notes on your own, that pressure doesn't exist. You also need to learn how to pick out the important information within the clutter of text. Don't be that person who literally rewrites the entire textbook in your notebook. Not only will your hand die, but your notes will also be useless.
5. The textbook may have extra information that may help you
Sometimes it is difficult to find a definition for a word that you may not know in the context of your subject, or you find conflicting definitions. You can then refer to the glossary in your textbook to ensure that you are learning the right definition. Textbooks usually also have summaries at the end of every chapter as well as practice problems. They may even have online resources to further aid your learning.