I'm pretty sure turning the egg keeps the developing chick from floating up to the shell then sticking and dieing. I turn mine three times a day. Hopefully some one with a little bit more knowledge will put in some input.
It's exactly that, the developing chick would stick to side of shell. A broody hen feeds a few times a day, each time she returns to nest she rolls the eggs about. It's nature. On her 18th-ish day; once she feels the chicks moving in eggs under her she no longer leaves the nest or moves eggs. We duplicate this natural occurrence by raising humidity and not turning eggs last three days, called lock down.