1. Make the most of class time.
When you read ahead of time, you're more likely to understand the organization of the lecture. You'll be better able to figure out what's important and what isn't (and thereby take effective notes).
2. Understand the topic and what you don't understand.
If you've read beforehand you can focus your attention on filling gaps in your understanding by paying more attention during some parts of the lecture and by asking questions.
Most classes require at least some participation -- answering questions and participating in class discussions. It's easy to participate when you know the topic. Reading beforehand helps you to know the topic and have the time to consider your perspective and opinions.
4. Show off.
Reading before class lets you show that you've read, that you care, and that you are intelligent. You'll be able to ask intelligent questions and participate in a way that demonstrates preparation, interest, and mastery of the material.
5. Participate in group work.
Many classes require group work, often in class. If you've read, you're ready and likely will not mooch on your classmates, or benefit from their hard work. In turn, if you've read you can tell when the group is taking a wrong turn.
6. Show respect.
Reading ahead of time shows respect for the instructor and interest in the class. While instructors' feelings shouldn't be the primary motivator of your behavior, relationships with faculty are important and this is one easy way to get your relationship with your professor off to a good start.