I can reason about any API or specification. But when I'm trying to get a different, compound, and higher-level task done, its quite a bit faster and less distracting if I can rely on someone else to have already distilled what I need (into a library, cheat-sheet, tutorial, etc).

Similarly, I've seen LLMs do things like generate clients and scripts for interacting with APIs. But its a lot easier to just hand them one ready to go.