The reason it attracts dirt to the rear of the car is because of the aerodynamics, and co-efficient of drag, (of which a golf has a better rating than a porsche 911) when going along a vacuum is produced at the back of the car, this "sticks" the car to ground better, and allows it to go faster, with a higher amount of safety.
There is no need for it, unless you want to use it to clean the shyte off the screen.