I like how in JJK it's a chance to create variation in power levels while also allowing an in-universe reason to expo-dump about a character's power.

It makes a shonen trope feel like these genius characters aren't just giving away all their tricks for shits n giggles. But then sometimes we just get a total lack of explanation and that's fine too because it's more fun, and the character doesn't need to (cough Sukuna)