I use Claude pretty extensively on a 2.5m loc codebase, and it's pretty decent at just reading the relevant readme docs & docstrings to figure out what's what. Those docs were written for human audiences years (sometimes decades) ago.

I'm very curious to know the size & state of a codebase where skills are beneficial over just having good information hierarchy for your documentation.

Skills are more than code documentation. They can apply to anything that the model has to do, outside of coding.