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Particularly in the case of a semaphore or mutex.

One function of blocking is to give up the CPU when you can't do anything, and to queue for a resource.

It would turn simple code into something like the following, but the pastabilities are endless:

gotit=0;
waitforit() { gotit=1 }
func() {
...
getit(resource,waitforit);
while(!gotit) sched_yield; // heat-up, drain battery,etc.

Of course there might not be callbacks, but check-if-done functions,

while( !areWeThereYet(destination) ) burn();

Of course if it is truly asynchronous, the callback function will have a parameter specifying the callback which needs to be called when the first callback is completed.

I could live without blocking, but I could also live in a post-apocalyptic world as well, but it wouldn't be any more easy, simple, or pleasant.