A very simplistic example is a window. You could make it with a bunch of polygons -- the frame, the glass, the sill, the lock. Or you could have a two polygon model, one polygon textured to include the glass, frame, and lock, and one polygon textured like the sill that juts out from it. The latter is how it would be done in a 3d game environment, but the former is how it would be done in a prerendered situation like an isometric game or a movie.