Over time headlights become yellow and foggy due to oxidation and effects of UV radiation from the sun.
They may also become scratched and pitted from everyday driving.
From a safety perspective, the worse they get the less you can see ahead of you at night.
From a cosmetic perspective, if the rest of the car is well taken care of and looks amazing, yellowed foggy headlights degrade the look and value of the vehicle.
In some cases we can restore headlights with a light polishing. For severe damage full reconditioning is required including wet sanding, buffing and polishing.