Comment by OtomotO

Because of I/O.

Without any kind of async (this includes green threads) you run out of (OS) threads very fast.

This is not a black/white decision whether Async makes sense for every API all the time or never.

It's a solution to a specific problem that occurred (and still occurs) a lot.

The article was just about the syntax though, because they are still using asynchronous programming via coroutines (or goroutines ;-))