The inclusion of satisfying set up and pay off in your story means your audience will find it so much more entertaining. The set up is early story beats that are part of the story in general, but then are pulled upon to be relevant to the climax of your story later. It means your audience feels rewarded for paying attention, and included in the story process because those early set up notes are put in just for their enjoyment.
A three act structure is like a framework for your story, and is how you move your plot along in a controlled way that keeps your story organised and making sense. However, as with most things there is fluidity to it and if a three act structure doesn’t work for your story that’s okay, but if it does work, and it does help you, then this is a good guide on how to implement it and why you might want to.