I eventually wrote that. just take the example and look at a golf ball. That rough skin stirs up the air around it in order to delay the air separation from it's surfce downwind. A spoiler does this with a smoother flow of air because in theory, you can control which end is pointed foward. It's a means to control the airflow vorticies created from friction and form drag so that the air converges better aft of the vehicle. I'm sure they put enfaces into airflow, but the determining factor though is of coarse image of the vehicle...
I just enjoy seeing people mod their cars, and adding personality. I also don't think that a different spoiler will be so adverse to the design that it will change the performance so much.