Headlight lenses are polycarbonate with a factory coating on the outside. Sun destroys that coating, the plastic underneath oxidizes, and the light stops going where it is aimed.
A yellowed lens scatters light instead of projecting it. You lose distance down the road and you throw more glare at oncoming traffic, and because it happens over years most people never notice how much they have lost.
Polishing alone removes the oxidation and the remaining factory coating with it, leaving bare plastic exposed to the sun. Without a new sealant on top it will cloud again quickly. The sealing step is the part that decides whether the work lasts.
Cracks, internal moisture and delamination inside the lens cannot be polished out from the outside. At that point the honest answer is a replacement housing.
If it has been hit and it also needs mechanical work, it does not need to go anywhere else.