To understand why it is not necessary to leave any lights on when parking on a road during the day, it is important to consider the rules regarding the use of lights and parking.

- **Use of lights during the day**: Vehicle lights should be turned on during the day only in specific situations, such as when there is little natural light, when passing through a tunnel, or when driving on reversible or additional lanes. Under normal daylight conditions, it is not necessary to turn on the lights when parking.

- **Parking on roads**: When parking, it is essential to ensure that the vehicle does not obstruct the passage of other vehicles. There is no mention of the need to leave lights on when parking on a road during the day, as long as the lighting conditions are adequate.

Therefore, if a driver parks their vehicle on a road during the day and the lighting conditions are normal, it is not necessary to leave any lights on.