What's the point using callbacks if they're only called once?

Can someone explain what's the point using a callback if it's only called once?
Concurrency?

Don't we barely need concurrency if all of our program is waiting for a single request result? (That's often the case for me.)

Moreover, pyramidal code obstruct readability.
For me, await is a gift (not always, but very often).

So, how are you handling asynchronous code most of the time?
I'd be happy to find out great design patterns I may not know :-)