Which factor is NOT a reason to use headlights during the day?

Using headlights during the day is largely about ensuring visibility and safety. It's important to use headlights in situations like rainy weather, foggy conditions, and low visibility conditions, as these factors can significantly reduce how well other drivers can see your vehicle.

On sunny days, however, visibility is generally less compromised, especially when the sun is shining brightly. In these conditions, the use of headlights is typically not necessary as the natural light provided during the day is usually sufficient for both the driver and other motorists to see each other clearly. Therefore, on sunny days, there isn't a compelling reason to have headlights on, making it the exception among those conditions where headlights would enhance visibility.