I have one and have never needed it. I have them in all my large cars (18 wheelers) and in two different accidents the driver showed the capture to the law and we were home free. The four-wheeler drivers were not happy. BTW, all commercial trucks and busses have them now. They are required by law and can be a life saver for potential lawsuits as it is very hard to argue with recorded facts. They can also sink your boat if your driver is doing something stupid.
Companies can also have a driver facing cam so they will know what he/she is doing behind the wheel. This has not yet become required, but it will as government controls continually increase.