The use of translighhts
Why use backings, rather than simply shooting everything on location? The answer, as to so many questions in Hollywood, is control. Control over lighting conditions, the time of day, and the weather. The ability to do thirty takes of a scene against a perfect sunset, and still have the option of coming back after seeing the dailies for twenty more.
So instead of shooting everything on location, which requires permits, crowd and traffic control, and ability to squeeze huge grip trucks onto a residential street without annoying the neighbors, a compromise is reached. Shoot the exterior shots on location in the shortest possible time, but come back to the stage set for the interiors.
This is where the backing becomes critical - the backdrop brings the location onto the stage. Ideally, every production would use customer backings shot at the exterior location to provide perfect visual continuity between the exterior and interior shots. In practice however, sometimes rental backdrops of generic locations are often used to achieve the same general effect.