Because it's faster.
If the WinAPI docs are solid you can do things like copy and paste pages of them into Claude and ask a question, rather then manually scan through them looking for the answer yourself.
Apple's developer documentation is mostly awful - try finding out how to use the sips or sandbox-exec CLI tools for example. LLMs have unlocked those for me.
But you have to check the answer against the documentation anyway though, to validate that its actually correct!
Unless you're just taking the LLM answers at face value?