Horizontal Bulge
The horizontal bulge is designed to reduce the amount of direction error on heel and toe impacts. If impact is made on the toe of the club, then the face will open up at impact and the ball will begin to fly to the right of the target (assuming a right-hand golfer). The influence of the bulge is imparting a counterclockwise spin to the ball, which helps it curve back to the left or back toward the intended target.
The opposite scenario applies to shots hit toward the heel. The ball will begin to the left (assuming a right-hand golfer), as the clubface will tend to close. The bulge will then produce a clockwise spin to the ball to help it curve back toward the intended target line. There is a term used to describe this occurrence, it is best known as "horizontal gear effect."