Why do some light fixtures use multiple bulbs?
We use artificial light to light up things.  The brightness of a thing depends on how many photons are falling on it.  Doubling the number of bulbs actually does double the number of photons.  That makes the thing visibly brighter.

One basic method of dimming a room is to have 2 switches, each of which controls some lights.   

More bulbs will not throw the light any farther.  What will happen, however, is that faraway things will get more light with more bulbs.  For instance with a single bulb, it might get 5 lux... with 4 bulbs it will get 20 lux and thus be more visible.