Arched benching (especially with a retraction of the scapulae) allows a greater use of the lower fibers of the pectoralis major (chest) muscles. Not only is this pushing angle likely safer for the glenohumeral joint of the shoulder, it’s also conducive to the great use of the larger mass of lower fibers of the pectoralis, which creates a more forceful lift without sacrificing as much safety as a flat pressing position might.