Part of the problem is that none of us have homes big enough to really capture, in true proportion, the real scales of the great outdoors! We must resort to a variety of modeler's tricks to fool the eye into thinking it is seeing more than it actually is.
Landscape painters have a whole arsenal of techniques to make oil on canvas look like a scene from life--even those with an impressionistic style can make a scene look "real" if they can successfully capture proportion, color, shade, perspective and other cues that make a scene look real. Photographers can use a similar set of tricks to fool the eye or modify the view of what is there to change the mood or feel of a scene.
Much depends on an individual railroader's emphasis--if you don't care much about making a scene look realistic, you won't bother with such trickery. If you do, then you have probably given it at least some thought. If you're a beginning modeler, and the thrill of that loop of track on the plywood is beginning to wear thin, learning some scenery tricks can help maintain some of that magic for you.
Personally, I am an urban modeler--I like tight, crowded cities, busy switching yards and industrial settings filled with grease, grime and human artifacts. But even an urban modeler MUST use these tricks--otherwise our task becomes much more difficult.
Here is a relatively mild example of what I am talking about:
The two shots above are of the same scene, shot from about the same perspective. However, the second shot LOOKS larger. The scenery makes it look more like a "real place", as opposed to the bare foam and MDF of the first shot. The backdrop makes the background look like sky, rather than a wall. (Okay, if we ignore the gap in the backdrop panels next to the water tower--but the fact that most of the sky is blue makes it easier to do so!) And the bit of tree visible behind the factory implies that there are things behind the factory, even if we can't see all of them, other than the wall of my layout room, two inches away. Finally, attention to lighting makes the scene look more like an outdoor scene in color and tone than a shot of the inside of my garage.
A backdrop helps a great deal--and can conceal many lies. Anything that prevents viewers from seeing too much of your layout at once makes it look much bigger--centerline backdrops, around-the-room configurations, high mountains, tall buildings, etcetera. If we can't see it all, we assume it has to be big!