I've been working as an editor on movies for 17 years now and one of the most common questions I get from people who don't work in film is: "What exactly does the editor do?"
Well, I'd say my primary job is to unfold the story in an entertaining, engaging manner for the audience. This is comprised of countless details and decisions. If this shot lingers just a little more, it will allow the audience a moment of reflection to really be there with our protagonist and empathize with what they are feeling. If this shot is just a little shorter and is moved after this one; it tells this story point in shorthand in a scene that is necessary for the overall plot, but not engaging in and of itself. This actor's reaction isn't the most genuine so perhaps favoring the other actor in the scene will keep the audience more suspended in the reality of the film. You get the idea.
Check out this awesome video I came across that illustrates these points in a really fun way.
If you are curious to read more about the art of editing from some of my peers, check out this article.