Good Mornings, as far as I understand them, are supposed to basically strengthen the muscle groups that need to be to have proper deadlifting form.
I noticed that with my deadlifts, I have trouble keeping my shins vertical because my hamstring is too tight/not strong enough along with my hips being a little short to allow my butt to move backwards enough to the point where my shins aren't exhibiting any motion but are completely vertical. What ends up happening is that when I am on the way down, the bar is not moving in a completely vertical manner because I am having to go out in front of my knees a little bit. That's because my shins are not vertical, and when that happens, you tend to utilize your lower back more which is a no-no and can be detrimental.
It's very easy to allow yourself to push your knees forward and too far in front of your midfoot when doing deadlifts. Good Mornings are supposed to alleviate that because you are focusing in a bit on the part that is essential to proper deadlifting form in the downward movement, ie keeping your shins completely vertical and your leg past your knees immobile.
Someone please correct my if I am wrong.