To understand why it is not mandatory for a vehicle to have **headlight washers**, but it is recommended, it is important to consider some aspects regarding visibility and the maintenance of vehicle components.

- **Visibility while driving**: It is crucial that all the vehicle's glass surfaces are clean to ensure safe driving. This includes the windshield and rear windows, which are kept clean with the use of windshield wipers and rear window wipers. Although headlight washers are not specifically mentioned, their function is similar: to keep the headlights clean to improve visibility.

- **Windshield wipers and rear window wipers**: These devices are essential for keeping the glass surfaces clean and ensuring good visibility. Although it is not mentioned that headlight washers are mandatory, their use is similar to that of windshield wipers, as they help maintain the headlights in good condition, which is recommended for better illumination.

- **Vehicle lights**: Lights, such as high beams, are mandatory in certain conditions to ensure that the vehicle is visible and that the driver can see well. Keeping the headlights clean with headlight washers can improve the effectiveness of these lights, even though it is not a legal requirement.

In summary, although it is not mandatory to have headlight washers, it is recommended to keep the headlights clean and ensure good visibility, especially in low light or bad weather conditions.