There are so many scenarios for debugging APIs that don't fall into the "I control both ends, so I should be able to see everything." camp. SDKs, poorly implemented or documented APIs controlled by a 3rd party (frequently even across a company where the people producing are far removed from the consumers), differing environments (works locally, not in staging, etc), UI-less machine-to-machine interactions (debugging webhooks is especially annoying) and many more.
If you check my bio, you'll see my bias toward these problems. But there are a lot more of them and a lot more variants than you give credit to.
