I have also been advised in the past(by Nikoluski), to actually turn my back to the mirror. As other have said, It’s a tool that won’t always be there. And I’m comparing this to sprinting too, and the fact that when i can see my shadow right beside me, for some reason it is easier (to hold good form).
But shadows are not always there.
Maybe the shadow analogy is BS :o, but I suppose the point is to learn your body intuitively, so things become automatic?