When you see a band play, there are usually black boxes on the front of the stage; if you see them from the side, you can see that they're roughly wedge-shaped, more or less like this:
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Those are speakers, and they're called "monitors"; usually the drummer will have one or two monitors of his or her own back there. They're there so the band can hear themselves properly. But aren't they up there with the amplifiers?! True, and that's exactly the problem: Everybody's standing nearest his own amplifier, and the drummer is in the midst of a bunch of drums. The singer is probably right in front of the drums, but the singer doesn't have an amplifier at all: The voice is coming out of the PA speakers, usually off at the side of the stage. Everybody needs to hear the drummer, and the singer needs to hear his own voice if he's to stay on pitch -- and this and that; the point is, the main PA speakers are fine for the audience, but it's different on the stage. Monitors are important.
Have you ever heard a band in a bar, where the singer was woefully off pitch the entire evening? Chances are the monitors were broken, or weren't loud enough, or weren't even plugged in. All of these states are common on that level. The poor singer may have been making the best of an impossible situation. Then again, maybe the poor dumb bastard couldn't stay on pitch to begin with. That's pretty common, too, at least in the bands I've been in.
I seem to recall that the band usually gets a different mix than the audience does, but I've never played anyplace where the monitors actually worked, so I can't say for sure.