Have you ever noticed your paddle blade fluttering through the water as you're taking your stroke? This is common with paddle blades where the power face is completely flat. Flutter is not only inefficient, but it can also lead to stress and tendinitis in your hand and wrist.
Dihedral blades have a raised edge align with the paddle shaft leading through the middle of the blade and flattening just before the blade's top edge. This edge, much like a boat or board's bow, splits the water as it comes in contact with the blade allowing the blade to move smoothly through the water.