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.