The daggers and rudders offset the lateral force of the sails. The difference between the rudders and the boards though are that the rudders are on a pivot. By raking aft, the Center of Effort (CE) moves aft, this loads the rudders up. If they are adjusted right and you have little to no helm, you do not increase drag b/c you're not having to pull on the stick to keep the boat going straight. Just like the sails, the boards have a CE and all the loads have to balance some how. Raking the rudders under or out just moves their balance point closer to or further from the pivot point of the rudder and this is what gives you the "feel" of weather or lee helm.

By raking aft, you can (think about going upwind on a sailboard) improve your upwind performance to a point. If it's REALLY windy, rake will also help your "survivability" downwind.

Adjust the mast to the "optimum" for the platform and adjust the rudders to fix any helm issues.