Front wheels are 335mm wide, the goal of the fenders is to move the air horizontally. Audi made the last R18 tub so boxy and narrow for the same reason, and their LM pods pushed the air down as well to help keep it from going over the big honkin holes.
The fenders also have extensions to keep air from spilling back over the sidepod until they want it to ahead of the rear pods. Mirrors were mounted to them to help with legality I believe.