The whole point is to block. Non-blocking variants are in general unreliable. And if you don't block and get a buffer overflow, you make your application unstable because you're constantly spinning in the loop waiting for the buffer to be available thus eating cpu power that could be spent on consuming messages from the buffer.

I can't think of a single situation where you wouldn't want to block (unless blocking was more expensive than a busy loop). If you produce an element and the buffer is full and you don't want to block then what will you do with it? You can only discard it, you can't store it anywhere because you risk falling off a cliff as you are producing elements faster than you can consume them. Blocking is a way to slow down production or consumption in a graceful way.