What purpose does it serve? Changing the color, or opacity of your tail lights can give your car, truck, or SUV, a "custom" look and feel. It can attract attention too and serve to offer that "WOW" factor many people look for in a vehicle. Most people do it for looks. So while there are variations to tinting the tail lights that can be illegal, this application is actually one that enhances the overall performance of the tail light and is far better than the factory look.