Everyone hate the sound feedback. I guess no one likes the intense buzz.
The feedback is due to the amplified sound from the monitor speaker is picked up by the microphone again and re-amplified. If it is cycled, it will produce constant harsh noise.
When the whole band asked the engineers to turn up their volume, the feedback could not be avoided. But if turn down the volume, then musicians are hardlly to hear themselves, and it will also affect the performance.
At this point, IEM can effectively solve this problem: the audio console directly sends the sound to your IEM, and no feedback is generated.