Ultraviolet light creates chemical changes in materials, and it can bleach colored surfaces over time.
Most light sources contain UV, including sunlight, flourescent, and incandescent bulbs.
Because they receive less illumination. Look at a room that's freshly painted, and you'll notice that the corners look darker. If a spotlight is projected against the floor of a simple computer-generated room, the rest of the room will remain pitch black. However, if the software takes each part of the illuminated floor and treats it as a light source, then that spot of light will dimly illuminate all the surfaces in the room. And, every illuminated spot on the room's surfaces must also act as a light source, which then illuminates all other surfaces.