I actually should have said:
If you set compressor thresholds too low, gain is still the same, but ultimate level will be reduced, so the system will feed back at a lower output level when gain is increased.
Geri O'Neil explained a typical compressed monitor mix problem already, before I checked back, but as an extreme example, try, or think of the following:
With limiter post graphic, threshold set at a level that will not cause any gain reduction, ring out the monitor with a vocal mic, get as much gain before feedback as possible without butchering the sound. Whisper into the monitor, it should sound like a whisper. Yell, it should sound like a yell. Increase the gain by 3 dB, it will feed back, bring it back to the stable EQ/gain setting.
Now set the limiter to 10/1 ratio (or greater)fast attack & release, and the threshold to -60 or whatever the minimum. A whisper will still sound normal. A yell will result in little more level output than a whisper.
Now, if you were to increase the gain by 3 dB to "compensate", the system would start feeding back, but the limiter will squash it to a whisper level. More gain, but no more level.
Obviously, this won't work for the performers! I do employ heavy limiting when ringing out unknown microphones, saves the ears and HF drivers while you learn how the new mic behaves.
Art