Most writers have a natural instinct when it comes to story structure and plot. We do so much reading, or we see so many movies, that understanding pacing is just part of our makeup. Even new writers, still learning their craft, have this instinct.
We know that if we have action scene after action scene, eventually we need to have a calm scene that gives us the chance to breathe. We know that there needs to be those moments of reflection, looking at what has happened in the past, but if we spend too much time dwelling on the past, that explosion will be needed to get things going again. And there will also be those moments when a character decides to risk all and just go for it—damned the consequences.
As writers, we follow a story structure with the instinct of knowing where the rise and fall in the action needs to be. However, when something is off in the pacing of a story, that's when all the discussions come out about one of countless number of models for story structure and the analytical tools associated with it.
As a developmental editor, it is my job to look at story structure and pacing from the macro and micro levels. If pacing is off, for whatever reason, I delve into the mechanics of a manuscript and tear that structure apart to find out why things just don't work. Much of what I do on this particular front is instinct, simply because every single story is different. However, there are some commonalities within the beats of a story, which has given rise to models like the traditional 3-act structure.
It's time to start dissecting the mechanics of some of these models, just so you, as a writer, can see why editors use them so often within our work.