I'm not sure either but he was right...I tend to be content with myself at the beginning until I view my work in the mirror and and discover horrors :)

I Don't have the space to set it up, but I had to remind myself to look in one, I should probably do the same and buy me a small one.

btw, there are times when you cannot use a mirror, like in landscape,
Also I think its a useful tool, but there are times that when it is necessary to be able to judge correctly without it.
Sometimes when you are enjoying the process of painting a lot it seems from my end that using a mirror would be like a pause to the process.

Small update
 

She's close enough at this point, I'll have to work more on her hair, dress & jaw, might add a few finishing touches here and there then declare it finished.
Until next time.

I hope you all like it.

Many artists also own a small hand mirror. The reason I got the stand-up mirror and placed it about 10 feet behind my easel is that it is easier to glance over my shoulder than to reach for the hand mirror, turn around, position myself and the mirror, and then re-hang the mirror and go on working. But I keep my hand mirror handy hanging on the frame of my easel. Also, another big plus for positioning a mirror behind your easel is that it effectively doubles your distance so far as viewing your work in reverse. If your mirror is 8 feet behind you, it has the effect of letting your view your work from 16 feet.

I haven't thought about it like that...I've always walked as far away as possible and just kept staring at my paintings...
That is totally very very useful thank you Richard.