In the event that you are in a car accident, the footage can be used to determine you were at fault. Because dash cam footage can legally be used as evidence if the other car is at fault, it can also be used as evidence if you are at fault.
Along with car accidents, dash cam footage can also be used as evidence for any kind of traffic violation. If your dash cam has GPS tracking, it will be able to determine your speed at any point of the recording.