Most US colleges will grant you credit on early courses in subject material if you score well enough on the tests.

If you take enough AP tests and pass, you can pretty much start 1/2 year ahead of your peers or more. That can save you money and/or time, or let you get on with advanced studies.

And further: high school AP courses prep you to take a standardized 'AP Test' in that same subject, which (depending on the subject and college) may even give college credit, allowing the student to skip otherwise-required classes at college.