One, if you are doing well in college prep courses, you may want to challenge yourself more by taking an Honors or AP course in that subject.
Two, the exam is graded 1-5, and if you earn a 3, 4, or 5 on the AP exam, many colleges will give you college credit or advanced standing. For example, if you earn a 4 or 5 on your AP Bio exam and enroll in George Washington University, you get to 'skip' their beginner Bio course. Here's GW's listing of what AP exam grades they accept: http://www.gwu.edu/apply/undergrhttps://undergraduate.admissions.gwu.edu/advanced-placement-ap-credit
Look up any college's policy here: http://collegesearch.collegeboard.com/apcreditpolicy/index.jsp (or do a search on any college's website for "AP credit")
Here's great advice from the Admissions office at University of Virginia http://uvaapplication.blogspot.com/ about course selection:
"If you're a junior or sophomore reading this, this is a time to think about balance. You want to decide how many of these top courses you can handle while still maintaining good grades. Don't sign up for every top class you can fit into your schedule if that means you'll be struggling with Cs. Similarly, don't shy away from challenges just to have all As. Obviously, it's nice to see all As in top courses, but we would rather see you sacrifice that A and get a challenging course into your program over taking the path of least resistance."
"We will look at the program you have chosen in reference to what was offered at your school. We wouldn't penalize a student who was only offered 5 AP courses at their school. We would look at the program they put together in light of their options. Ideally, as you've been able to make choices about your program, you will have worked in those advanced courses so that by senior year, you are close to college level work. You want to show your schools that you are ready for the next step; that the transition to college isn't going to rock your academic world in a bad way."