Your notes are your record of the class:
College professors use lecture to supplement the textbook. While you have required reading, rather than repeat what's in the book, they will provide additional and new information that isn't in the text. If you don't take good notes, you will not have the material to review while preparing for tests. Even if they cover the same topics as the textbook, they will provide additional examples and views that aren't available in the text, but that may be on the test.
You can organize your way:
As you are learning, the information will probably seem disorganized and lack connections. When taking notes, you begin to learn the professor's style, and it makes it easier to see patterns in the instruction. Tailoring your notes can help you organize the information and see connections that were obvious at first.
Notes can provide insight on how you'll be tested:
By reviewing your notes from several class meetings, you'll be able to see what the professor emphasizes and that should help you know what might be on the test. Typically, the more time is spent on a topic, diagram, table, etc., to more likely that it will show up in some form on the test.
Taking notes helps you stay focused:
It can sometimes be hard to focus on what is happening in the class, particularly if it's not a class that piques your interest or relates directly to your major. Taking notes helps you keep your mind on what's being said, and helps you stay awake and alert.
Most importantly, effective notes help you learn:
Just writing something down increases the chances you will remember it, even without seeing the note again. And then reviewing the notes later in the day and again later in the week gives your brain a chance to really lock down that new material.