If what you want is the "Why", you're right it probably won't get it right. It'll just give you baseless speculation. But if what you want is the "What" or the "How", I think it can be useful.

The code is the information. If you need binders of info then it wasn’t codified

Yes, it would take a long time for humans to remake a documentation from the codebase itself, but AI could do that much quicker and with close to perfect accuracy, Code is superior to documentation as it's the direct intended behavior (including bugs), in a sense, we won't even need documentation in the future, except maybe to lower AI costs.