Let’s say the course is offered at your HS, what are the reasons for self-studying for the AP exam? Given the score is a 4 or even 5, do they count as a course for your school? (I know you can write that you got a certain score on your college application)
• Personal satisfaction
• Potential college credit
That’s about it as far as I can tell.
It’s a way to capture outside validation that you have mastered a body of knowledge. If you know most of the subject matter and want that to be registered (for college credit, etc.), then self-studying to fill in the gaps so you can score well on the test is a way to ensure you have thoroughly covered the subject matter and to get “official validation” for it. Some people also like the award designations (“AP Scholar”, etc.) and collect points towards award levels.