My understanding is that awaitable is introduced for the native coroutine use cases. Native coroutine objects follow the generator protocol (.e.g, send(), throw(). etc). But different from generators, coroutine's send() never takes value, i.e., it always send(None). I think that requires the _await_() function to return iterator kind of enforces this: next(t) is equal to t.send(None) if t supports the send() method.