I have little experience with sound systems, but perhaps an incompetent opinion or two will get some discussion going.

My "cheapie" sound book, Sound Reinforcement Handbook, Gary Davis & Ralph Jones, Yamaha Corporation, 1989, ISBN 0-88188-900-8 (about $35 US) lumps compressors and limiters together, depending on how they are used. In either case, the device limits the amount of increase in "delivered" power as the input signal increases beyond some selected level.
Used as a limiter, the intent may be only to protect the speakers - as when someone drops a microphone.

The "extreme" use of compression is in voice/announcement types of PA systems, especially in noisy environments. By using a lot of compression, all of the signal can be delivered at essentially "full power" regardless of the dynamics (fluctuations in power level) of the input. The example given is the announcer at a "moteor race track."

It would appear that use of some moderate amount of compression could help solve the "mike distance" problem, - the variation in output as performers move around the microphone; and could be used to push more of the output dynamic range above any crowd noise threshold.

My favorite festival has gone through a long period (15 years) of degradation of their sound by hiring "experts" whose answer to everything was "more amps." This year they finally realized that "the louder the sound system, the louder the crowd, which means you have to get a louder sound system, which makes the crowd louder."

They finally cut back to a level this past year that sounds good if the crowd keeps (somewhat) quiet, and things were greatly improved.

Sophisticated limiters, compressers, and "companders" probably are most frequently used in recording/playback, where there are some well known "thresholds" like tape hiss (background noise) or needle jump/tape saturation limits.

Auditorium sound reinforcement possibly is more complex, due to the uncertainty about - and variability of - the background, and by how much additional power is needed/acceptable as the room fills. This means you may need more "adjustable" features - and also increases the possibility of messing things up if you don't have someone with experience and a good attitude running the system.

Now, lets wait for someone who actually knows something...

John