When I was starting out as a screenwriter, I tended to write movies as if they were plays: a scene would begin when two characters entered a room, continue through their entire conversation, and then end when they were finished. I wrote them that way because I wasn't thinking like an editor.
Generally speaking, editors try to use the minimum amount of footage possible to tell the story. They're not likely to keep an unnecessary shot simply because it's beautiful, or to leave in a lengthy monologue for the sole purpose of enhancing the actor's chances of an Oscar. Their goal is efficiency, not indulgence. "Efficiency" isn't a sexy word. It might well sound anathema to the whole idea of artistic expression -- but it isn't. In fact, it can make for some truly beautiful and effective scenes.
FADE IN: On a college lecture hall. The PROFESSOR, a balding man in his sixties, walks up to the podium. He sighs and introduces himself to the class. As he goes over his plan for the semester, we can hear in his voice that his heart is no longer in this. Students eagerly raise their hands to ask questions but are dismissed with brief, vague answers. CUT TO next scene.
FADE IN: On a college lecture hall. The PROFESSOR, a balding man in his sixties, walks up to the podium. He sighs. CUT TO next scene.
Example #2, of course, is more efficient. There's very little that the longer version tells us about the Professor's state of mind that isn't fully encapsulated in that single sigh. (Plus, ending on the sigh makes it more noticeable than it might be otherwise.) But there's another reason that the shorter version is so much better.
Here's a professor going in to teach his class. This is his job, his livelihood, presumably his life's work. And we're not bothering to show more than a second of it. Implicitly, we're telling the audience that there's nothing in that lecture worth seeing. We're establishing the fact that this guy teaches, yet simultaneously establishing that this movie is not about his teaching. And on top of that, we're hammering home the emptiness of his experience. Guy goes in to teach a class. Whatever. Let's move on.
Example #2 actually comes from the movie The Visitor (written and directed by Thomas McCarthy), which is a model of efficiency throughout. While people often assume that small, character-driven films like this one will be ponderous and boring, it's quite the opposite; a great deal of information gets conveyed in the first ten minutes, and the plot takes several interesting turns before the thirty-minute mark. All of this is made possible by the relentless devotion on the part of McCarthy (and, of course, his editor Tom McArdle) to making sure that no scene is longer than it absolutely needs to be to move the story along and tell us what we need to know about the characters. There are very few "complete" scenes in the movie, but there are a great many beginnings or endings of scenes.
Most of the time, the beginning or ending is all we need. At heart, every scene in a movie needs to convey exposition and emotion: This happened, and this is how the character felt about it. The Visitor example above does both those things in about five seconds of screentime. Seeing the beginning of the scene, the audience is smart enough to hypothesize the middle and ending. Conversely, if we show just the end of the scene -- but that ending contains the necessary elements -- the audience will infer the beginning and middle. This is a good thing, because the job of the filmmaker is to make the audience think, not to do all the thinking for them. By engaging their imagination, we make the movie a deeper and more personal experience.