Speaking as a layman, not knowing much about the art of singing, I was curious, do great singers need monitors (either the in ear ones, or speaker ones) to stop themselves singing out of tune? Because it seems to becoming a common reason/excuse for either not singing live, or else singing out of tune "the monitors weren't working properly". I know Gemini made this excuse when were pretty dire at Eurovision.

I guessing it a limitation of alot of singers, that they just don't know how they are sounding, and wouldn't be able to sing well at all without the technology. Perhaps separating the truely good singers from the rest. I mean how is it some people are able to stand up and sing in a normal room, without any aids at all? The only other thing you could maybe put it down to is may the concert environment changes the sound, so you need the monitors to help you. Anyone knowledgable enough in the field to know?

When you are singing with a rock group in a large venue you ALWAYS need monitors in order to hear yourself properly,otherwise you will sing off-key,as your voice coming from the PA speakers doesnt radiate behind onto the stage much or reflect back off distant walls or ceilings.
This is particularly the case at higher frequencies.
Your voice is also being prevented from being heard on stage if instrument amplifiers are in use.

The musicians also require monitors to enable them to hear the vocalist properly.

Monitors also provide individual mixes of instruments for each group member when playing live or just the complete recorded backing track if it's semi-live or mimed.

Recently, on a performance by Will Young, he was interviewed afterwards (I think it might have been PI2 but I'm not sure) and he said, as he took the earpieces out, "I can hear the music better through them not through the speakers".