Anyone who owns fleet vehicles understands how maintenance and repairs are an important aspect of keeping the cars and trucks on the road so they can be earning the money needed to keep your business running. You probably have an employee who is directly responsible for assuring they are running well. Whether you hire an outside mechanic as needed or have a large enough fleet to justify keeping an in-house mechanic to perform the needed repairs, you're going to need to buy parts to keep the fleet running properly and safely.
Windshields do get chips and cracks in them over time, and especially with fleet vehicles simply because they spend time on the road than privately owned cars typically do. In some situations a chip or small crack can be repaired, but if there are multiple chips and dings or larger cracks, it makes more sense to replace the windshield.
Side and rear windows don't typically have the same amount of problems as the windshield, but they are glass and do break on occasion. Because they are tempered safety glass they break rather than crack so they have to be replaced rather than repaired. Fortunately, replacing side windows is not a difficult process comparative to other repairs, it simply requires buying the new piece and having it installed.
Spare Tires (and replacement tires for roadside emergencies)
Everybody who has driven a car understands that flat tires are a fact of life. Whether your driver is capable of changing the tire or you need to hire roadside assistance, the problem has to be addressed immediately to get the driver safely home. After the immediate repair, the tire itself needs to be repaired so a spare will be available the next time it happens. If the car has a full sized spare, the flat tire can either be plugged or replaced and go back in the trunk. A smaller temporary replacement tire typically needs to be replaced after one use because they aren't designed to handle extended mileage like your regular tires are.
Other parts your fleet is going to need are light bulbs and fuses, a new battery on occasion, fluid changes, and new filters. It's important to have a good mechanic to handle these jobs and a car parts store you can trust to provide top quality parts you know are going to work efficiently for your fleet.