Over time, the plastic cover on your headlights will wear down and turn yellow.
This both looks nasty and can also be a WoF fail as they don't work like they should.
New headlights can be extremely expensive, especially in later model vehicles.
A cost effective way to fix this issue is to remove the headlights, sand them and recover them with clear coat. This not only makes them look like new again, but also protects them from harsh UV rays well into the future.