Can someone help an old fart here? When I think of "API" I think of, say sections 2 and 3 of the unix man pages. Calling conventions basically. When I write a library I don't need a special tool -- the functions and data structures in my external .h files by definition define the API.

This tool is clearly for something related to that (how you call a remote web web service) and clearly people find it, and tools like it valuable. But I don't get it.

This isn't me saying "you kids are dummies for needing this thing" -- I'm asking the opposite: "I don't get it: what important things am I not understanding?"