It's about reification... Callbacks are an implementation of asynchronous processing each slightly different sort of like 'function calls / calling convention'

It's about reification... Callbacks are an implementation of asynchronous processing each slightly different sort of like 'function calls / calling convention' in assembler, while promises provide a reified interface that is easier to think about conceptually because you're thinking about what to do, not how to do it. The contrived example in SO can just as easily be decoupled while still using callbacks, however the promise based code 'looks' better because it more concisely describes what's going on in terms of 'english'.