Actually it is not primarily to increase the bonding area (compared to a simple overlap), but to adopt the parts thickness to the loading, so that the deformation remains constant over the length of the bonding joint, hence the shear stress in the bond is almost constant. In a simple lap joint you would have enormous stress peaks at the ends of the bond joint, and the center would not carry any loads.
You smoothly transfer loads from one item to the other and you proportionally transition the wallthickness from full at the begining to zero at the end of the joint.