2mojo2 has it correct. Many times, the mains cannot
cover the entire seating area on their own, either because their coverage
pattern doesn't include the entire audience, or there are obstructions in the way, like a balcony. Other times, like an outdoors concert, the mains simply cannot provide the sound pressure levels (
inverse square law, in an ideal world, a doubling of distance results in a 6 dB loss for a
point source radiating with no reflections, that's the simple definition).
So, to work around this, we hang speakers to provide "fill" for these areas. In my
theatre, we have two Apogee AE-5's mounted
stage left, and
stage right against the wall, and about fifteen feet off the floor. If I'm standing under the balcony, the horns providing the high's don't make it under the balcony, so we have a ring of JBL Control 5's as
under balcony fill. We use the delay on the outputs of the
console to not only align the Apogees to the
stage, but to align the JBL's to the Apogees as well so that the signals from the
stage, the signal from the Apogees, and the signal from the JBL's arrive at the same time (well, we have them a
bit behind each other, which is another discussion). Like Mojo said, our brain perceives these as a single acoustic event.