Photo backdrops add a feeling of distant scenery between the actual 3-D modeled scene and the cloudy sky. The area previously finished required very little (about a foot) of backdrop, while the area I'm currently working on requires more. Generally, I use them where the track, a road, or some other element runs pretty close to the wall, and the planting of lots of trees to soften the transition from 3-D to flat vertical is impractical, because the area is narrow.
