I had a similar thought when C# introduced async/await. "Why all this complexity? What was wrong with good old fashioned blocking calls?"

I don't know the answer, but I would like to. I think it has to do with the limit on the number of processes/threads that the OS/framework can manage. Once you reach this limit, using async/await somehow allows the OS/framework to secretly use your (technically not) blocked threads to do some other work while they are (technically not) blocked.