Setting the body and the bench in their proper positions along the left-right, up-down, and forward-back axes allows the body to operate in complete balance. With the bench supporting the body adequately, the arms and legs can move freely from key to key, from manual to manual, and among the stops.
If this balance is not achieved, pain will result as muscles compensate for the imbalance and are forced to overwork. Experiment with various positions of the bench and the body until this balance is achieved.