By covering a side or two (or three) of a glass tank does help reduce the stress and makes them feel more secure. It doesn't have to be an aquarium background - although they do look MUCH nicer. Depending on where your tank sits, they could be getting too much 'action' in a room.
I have all my tanks covered on three sides (geckos, frogs and mice) because it keeps my cats from being able to terrorize the critters.
Some put for safety so they wont stress out they feel secure some for looks that really depends on you. I for one put it for looks I like to make vivs pretty