I have to admit that, while I knew these documents existed, I was not aware they included as much information as they do: I was under the impression that it was merely an xml version of the original C headers to facilitate their parsing. Having a second look, it seems that it also covers information like array expected size and valid enum subsets for parameters, which is very valuable data for all players using the API (even driver implementors could use it to generate basic conformance tests).
Yes it is and most language bindings are generated using them. This also includes "OpenGL extension loaders" which are required in C apps too.