Glass cleaner is used on a vehicle to clean the windscreen and other glass surfaces, such as side windows and mirrors. There are several reasons why someone might choose to use glass cleaner on their vehicle:

To improve visibility: Over time, glass surfaces can become dirty, smudged, and streaked, making it difficult to see through them clearly. Glass cleaner can help to remove these contaminants, improving visibility and making driving safer.
To restore clarity: Glass cleaner can help to restore the clarity and shine of glass surfaces, making the vehicle look newer and better maintained.
To prevent streaks: Some glass cleaners are formulated to leave a streak-free finish, which can be especially important for the windshield, where streaks can be distracting and dangerous.

Overall, using glass cleaner on a vehicle can help to improve the appearance and functionality of the glass surfaces, making driving safer and more pleasant.