It has been said that having backgrounds provides a sense of security for the fish. The principle is much the same as a horse wearing blinders. The more restricted it's field of vision, the less that can spook the horse. Same thing with fish. Though, since many people put their tanks up against a wall, there is not likely to be anything moving around to stress the fish back there. So, in most cases it is more for us than for the fish. I just have a blue posterboard cut to size behind my tank. It looks much nicer than a bunch of wires hanging around back there (from my filter, heater, lights, etc).
\Dan