Answer: improving your college GPA.
AP is really a college class in high school setting. The AP test is basically a test of the knowledge that you have gained over the past year, the special thing about this test is in order for your college credit to count you have to get a three or above for most of the colleges.
The advantages can be broken down into two categories time and money. If you take an AP course it will cut down the actual college time that you have to spend, as a result early graduation which means this classes you take in high school can later be skipped in college. And all this will lead to a job which means more money.