The headlight glass is a crucial part of the headlight assembly. Headlights are external lights, contained in either a metal or plastic housing, that are fixed to either side of the car at the front of the car. The headlight glass makes up the lens of the headlight, forming a protective covering, which fits over and forms the outer surface of the headlight.

The primary function of the headlight glass is to protect the bulbs that make up the headlight. The bulbs in the headlight assembly are fragile and can get damaged if exposed to the weather, which can cause the bulbs to fail. 

The headlight glass ensures that the headlight is completely protected from the elements and also offers protection to the bulbs from minor knocks and scrapes.  The headlight glass also helps to reduce the glare produced from the headlights, which helps oncoming traffic.  

If the headlight glass is broken, smashed, cracked or is damaged in some other way and not fitting securely over the bulbs in the headlight assembly, you run the risk of water seeping in and exposing the bulbs to bumps and knocks which can lead to broken or short-circuited headlight bulbs. It's important, therefore, to replace the appropriate headlight glass when damaged to avoid damage to the bulbs. Replacing the broken glass in the driver's side headlight with the headlight glass will ensure you get a part that fits securely to your car's headlight assembly.