Something like this: whenever you're drawing you are working on a specific thing at a specific place on the drawing, let's say the shape of a particular shadow. That narrow mental focus might come at the exclusion of paying complete attention to the value of the shadow. Leaving the drawing alone for a while, or, looking at it in a mirror, or upside down, breaks you out of looking at that shadow as a matter of shape alone, and you notice that the value is off.
That particular thing happens to me all the time. Finishing a drawing for me is often about adjusting all the values into a coherent whole. Other artists I know don't seem to get so confused about values and keep them consistent as they go along. They, however, might get fascinated by some other thing at the expense of something else, and the fresh look, where the original fascination is broken, allows them to see the flaws and take care of them.