Background “plates” or “stereos” (not a reference to stereoscopic 3D, but to large-format stereopticon 2D slide projection), another type of still, enable the studio to create location scenes without leaving the premises, thus reducing the ultimate cost of production.
Using background images or slides, apart from static shots, allows the studio to create on-location settings without having to physically move locations, thus reducing production costs and enabling more efficient use of resources.