I have found that sandpaper is necessary in some cases but it can cause problem in others.
If your lens is scratched, pitted chipped peeling or has some obvious surface imperfection, sanding or replacement are your only options.
Depending on where you live and the invironment you drive in would have a bearing on the condition of your headlight lens.
The vast majority of headlights are not damaged in that way. Normally oxidation is the problem most of us face with thermoplastic based headlight lens. They will become opaque and can change color from white to yellow to brown.
You can sand this oxidation off but sandpaper will cut through oxidation, UV, layer and lens surface. It cannot tell the difference between them. You will damage the factory UV protective coating. Once you remove that, your exposed pourus lens will quickly turn yellow and you will never get them clear again.
Best way to go is to use a liquid deoxldizer that will only effect the oxidation. Your UV layer will be left intact and your headlights won't sustain the type of damage abarasive compounds and sandpaper can cause.
My car is 4 years old, My lens were not damaged, they were pretty smooth, however the lens had a solid coat of oxidation over them, I used Pittman's Original One Step ALR on mine, I bought it online. It cut the oxidation off the lens as I was putting it on. Took me less than 10 minutes to do the headlights taillight and marker lights. They look great. One thing to keep in mind is that oxidation returns. It does not go away forever. You will have to clean your lens again at some point in time. Once you start sanding you will have no other option but sanding or replacement when they turn yellow. Use a non abrasive acrylic lens deoxidizer.