It's only functions that "block and wait" that it makes sense to be async - the point being that blocking and waiting doesn't use up CPU cycles, so you might as well free the thread up to do other stuff. If a function issues a socket write, then continues utilising the CPU while also periodically checking if data's available to read, it's effectively doing manually what async does for you, though I'm not sure there are too many real life examples of such functions. But pre-async it was certainly common enough for block-and-wait functions to tie up a thread and hence execution of programs with limited multi-threading (even today, GUIs often require all events and updates to be processed on the primary thread).