[neilc]

I think the obvious reason is that understanding why the API works as it does allows you to understand more about what your program is doing, and thus lets you write better software. For example, you can just think of malloc() as a magical "give me a block of memory!" API, which is perfectly sufficient for a novice who wants to write working programs. Alternatively, you can understand some (not all!) of the details behind virtual memory and how a typical heap allocator works. In the long run, that helps you write better software (e.g. by gaining an intuition for the relative costs of manual memory management, or debugging heap corruption).

So I think it's part of the difference between being a journeyman and an expert. That said, not all APIs are worth understanding, and you don't need to go into exhaustive detail with everything; it's a question of deciding what's worth learning in the long-run.