[Reply] Well there are different reasons why artists do that, flipping the image of course. But as a seasoned artist, I'd say the main reason would be proportional in scale for what you're working on. If you stay in one position while working on a piece then you're stuck seeing that and you think it's fine, right? But then you flip the image and suddenly everything is all lopsided and disproportional. One eye could be slightly lower the other, the arm isn't as long as the other, or the face altogether looks all swirly. By flipping the image from time to time an artist keeps himself in check and is sure of it's outcome. On paper this can be done by simply holding the drawing to a light looking at the backside to make sure everything looks fine. I've also met an excellent painter who kept two mirrors in his studio specifically his paintings. One mirror at the back of him and one in the front in the direct way of each other. His method was to often glance at the one in front so that he'd see the reversal of his painting in the back mirror to be sure everything was proportional. Artists like to be percise [in areas that call for such precision of course.]
Hope I helped-

Thanks for the Bday comment.

It was great to hear from you! Keep in the Word, pursuing that relationship.