It keeps the breast meat from drying out.
I did that once by accident (never cooked a turkey before, woke up at 5 a.m. to put it in the oven and I'm NOT a morning person!) and it tasted great - very moist breast meat.
Only thing is, since I did it by accident, I didn't turn it back over, and the poor thing didn't have that "nicely browned" look.
That's so the fats will run down into the breast so it won't be dry. The problem with cooking it breast side down is when you try to turn over steaming hot slippery 20 pounder, you're going to dump it in the floor, burn yourself, or both. Then there's the problem with the breast skin sticking to the bottom of the pan so you won't have a nice pretty table presentation with half the skin missing and dried out un-skinned meat. Better stick with the tried and true baking it on it's back.
This is an old trick, allowing the turkey juices to run through the white meat to keep it moist. We call it cooking it "tits-down" for a period of time and then "tits-up" for finishing it. It works, but that depends on what else you are doing with the bird.