one of the hardest things to do is set stage monitors the way everyone likes them. the vocalist needs to hear himself, as well as a chordal instrument to keep him in tune, and some drums for time. guitar needs different things, as does the drummer. IEM's allow every member of the band to get as much or as little of the monitor cue as they need. eliminates the need for on stage monitors, which are large, and loud, and often irritating. AND, it helps remedy a lot of feedback problems in the system. lets see, what else... oh yeah, saves your hearing, because you can set your own level. um.... lots more advantages. there are a few drawbacks depending on the system, but they are usually outweighed by the benefits. hope thats a little helpful.