Automakers used to make headlight lenses from glass, which limited the size and shape of lens they could produce. More recently, auto manufacturers began using plastic or polycarbonate headlight lenses, which also helps makes cars lighter and more aerodynamic.
Polycarbonate has its advantages but also its disadvantages — namely oxidation and/or discoloration. To the naked eye one cannot tell that these plastic lenses are actually a porous material that expands ever so slightly when it gets hot. Debris, together with a combination of environmental factors such as the sun (UV rays), smog, acid rain, car exhaust, and the harsh chemicals sometimes used to wash you car can cause the plastic headlight lenses to become oxidized, resulting in cloudy headlights. This is unattractive and also unsafe. Because of the degradation of the plastic lens, less light is passed through, resulting in diminished visibility during night driving.

In the past you either had to deal with these cloudy, foggy headlights — which posed serious safety issues — or purchase expensive replacement headlights, which could cost hundred or thousands of dollars.