The actual answer is that they act as air guides similar to the rudder on a boat or the tail of a plane. As air hits the wheel, the circular motion of the rim, aided by turbulance created by the spokes, forces air to follow the curvature of the wheel. The air is funneled between the legs of the fork (and the rear brake bridge), flowing over the carefully shaped wings of the brake pad carriers. This increases the stability of the bike.
It is not done much anymore, but sometimes during crits or circuit races you will see the old timers use pads or carriers without wings on one side of the bike. For racing clockwise they will remove the right side wings and view versa for counter-clockwise routes. This increases the handling in turns. In fact, back in the day, both Suntour and Sakai used to offer pad carriers with extra long, specially shaped wings that were designed just for crits. Suntour's were called "Sweet Wings," and Sakai's were named "Brake Critters." Ultimately the UCI felt things were getting out of hand and made rules that restricted wing "spans" to 12mm or less.