I usually reply, “Why so many light bulbs?”
Often, especially if your ceiling is low, it will require more than one loudspeaker to cover your audience evenly. A tall ceiling can allow a fewer number, even one, loudspeaker to cover an audience evenly, given that the venue’s sides are not parallel.
Sound, in particular higher sibilant frequencies, will emanate in a beam rather than an arc from a speaker. Ceiling speakers (typically those white 10″ round grilles in a suspended ceiling) are innocently notorious for displaying this phenomenon. Have you ever gotten stuck underneath one at a restaurant and realized everyone else is happy but you? To have gotten that application right, the ceiling should have been flooded with speakers and thus the volume of each reduced and further the dispersion more even. Flooded may be a strong word, but the point is that if you have an eight foot ceiling, you simply can’t evenly cover a 20 x 30 room with four narrow beams.
Conversely, even with a higher ceiling it is often necessary to divide the venue into zones, each with its own loudspeaker. If the venue has parallel or near-parallel side walls, it’s only a matter of time before the sound of a speaker spreads out enough to reflect off of them. This causes all sorts of problems, all noted in the last article on this page. To counter this effect, we introduce a number of speakers all with a shorter throw distance and thereby minimize the intensity of any reflections. The speakers are electronically aligned so that they perform coherently.