There are three elements to a solid position: Bone/Artificial Support, Muscular Relaxation, and Natural Point of Aim. Natural Point of Aim is simply where the rifle naturally wants to point upon shouldering it. The Natural Point of Aim is then adjusted for the desired sight picture, the relationship between sight and target. Properly adjusting NPA is important as it helps achieve a position where Muscular Relaxation is realized with Bone/Artificial Support, meaning less movement or wobble while executing the two firing tasks.
A novice shooter will usually use muscle to steer the rifle to the target while shouldering it; and thus, he will never realize NPA from which adjustment could be made; but having steered with muscle the shooter will need muscle to maintain aim, which wlll produce movement. The shooter cannot easily undue the poor position however since the brain wants the shooter to stay on the target rather than relax into NPA and thereafter adjust. This is why it is important to shoulder the rifle without initially looking at the target so it can not distract you to muscle the gun to it.
I don't know who first hypothesized NPA was important to good shooting. I do not know when the theory became fact. I do know that today it is seen by those who produce military marksmanship doctrine as one of three elements to a solid position as earlier mentioned.
