The promise in marketing are the expectations.
It's about uncertainty and the unknown. It's about not knowing entirely whats on the other side until you try it.
It's about having beliefs beforehand about how something will be or go without actually experiencing it yet.
It's the framing of something in your mind before it happens to you.
The question of promise, what it does, is it ties the question of "Who's it for?" and "What change are we making?" together and meets that person where they are at.
It allows you to ask yourself the question: Does my product or service line up with the promise I am making and does it line up with the expectations that have been set in their mind.
Because if there is not an alignment between all of these factors, the promise has failed.