They're designed to guide the wheel when installing it into the frame. Not that big a deal in the shop, but helpful out on the road. During a hurried wheel replacement (like in a race), you don't want the wheel to hang up against a brake pad. Pushing a wheel hard into the bottom of a brake pad will also move the calipers off-center. These wheel guides help to prevent all that.

 

Oh and they are handy for aligning / tensioning the brakes. Strap them against the rim with a rubber band then pull the brake cable through to the desired amount, then remove the rubber band and voila - perfectly adjusted brakes.

 

Avid Juicy disk brake pads have similar guides for use. Very helpful. You could take them off if you are weight conscious, but they do add a period correct asthetics.