Your headlights get foggy, mostly due to an external problem which can easily be rectified.
The plastic on your headlight gets dense and yellowish due to oxidation. Over time the protective hard coat breaks down with UV degradation, wear from abrasion etc. If left untreated the headlights will eventually develop small surface cracks, a condition referred to as crazing. The effectiveness of the headlight in terms of light output is greatly diminished, in some cases by as much as 70%. leading to reduced night time visibility.
These can easily be made to look ‘as good as new’ in under 30 minutes – a far more economical alternative than replacing the lens!
Additionally, headlight restoration extends the usable life of the headlight assembly and can be repeated. It is also considerably “greener” than disposal and replacement of headlights.