Comprehensive car insurance covers things that happen to your car which do not have an identified or traceable cause or at fault person. It is often required by dealerships and lenders, to protect their investment in your car until it paid for. Comprehensive and collision coverage round out the full coverage package when combined with the state required minimums.
If someone keyed your car, it would be covered by comprehensive insurance because of the anonymous nature of the damage. If it was broken into and the stereo stolen, comprehensive insurance is the type of coverage you need. Most willful acts of damage or theft are part of the comprehensive coverage, including theft of the car itself.
In many cases, window and windshield replacement are part of comprehensive coverage. If a rock got thrown up from the road and cracked your windshield, there would not be an obvious at-fault, but a cracked windshield is a serious problem, and insurance companies use comprehensive coverage to protect you.
The exact coverages of comprehensive insurance will differ somewhat from one company to another, as well as from state to state. The basic idea is that is covers damages and losses which do not have a well-defined fault, and that includes many aspects, including your personal property inside the car.
Comprehensive coverage may not be necessary if your car is paid for or starting to show its age. In situations where replacement or repair might cost as much as replacing the car, it is sometimes easier to simply accept that the car is disposable and save on insurance costs for comprehensive coverage.