self study out of:
- interest
- save money
- your AP class sucked and you still want to take the test
but I don’t know about your geographic/school circumstances, but not all schools will administer AP’s they don’t offer/are very reluctant and the process can often make it not worth it. but if there’s a particular subject offered at your school that you might not have the schedule space to take, but you have a lot of interest, it doesn’t hurt.
self-studying AP’s definitely isn’t a “hook”, but it doesn’t hurt your application and can somewhat show that you can handle challenging course and curriculums…but mainly AP’s serve to make your courseload more difficult to prove you can handle college level courses…so if they aren’t on your transcript in that form, their weight is lower.