SAT Subject Tests - Necessity and timing

Forgive me if this sounds wicked stupid, but if you’ve got a kid taking an AP course (and presumably, the AP test at the end of it), why do they need to take the subject test as well? For instance, if I tell my junior who is in IB bio HL right now that she needs to take the IB test, the AP test, and the SAT subject test, she’s going to think I’m nuts.

It seems nuts! Is there any rational reason you need to get the grade in the class, then take three additional standardized tests to let a school know you’re capable in the subject?

ETA; nevermind-I clicked on the link a pp posted and realized it’s required for certain colleges.

Le sigh, looks like she’ll be taking the subject SATs if they’re on her list.