In addition to making your vehicle look unkempt and unloved, cloudy headlights can be unsafe.
The cloudiness of the headlight lens reduces the power and effectiveness of the vehicle's headlights as it decreases the amount of light that can pass through the headlight lens. This significantly reduces the range and quality of night time driver vision.
Can cloudy headlights cause a WOF failure? Yes - [see here]
As mentioned, cloudy headlights make a vehicle look unkempt and unloved, and consequently affect a vehicle's value. It follows therefore that a headlight restoration can increase the value of a vehicle and in addition make it more sellable.