A dashcam's primary advantage is its unbiased account of events.
If properly configured and maintained, it can capture a wide array of relevant data:
Actual Speed: Some dashcams have GPS technology that records your speed at any given time. You can use this data to show you did not exceed the speed limit.
Road Conditions: The video can demonstrate that you were driving at a reasonable speed, for example, in a construction zone.
Traffic Flow: Footage may depict the flow of traffic, which can sometimes indicate that maintaining a speed slightly above the limit was necessary to keep pace with other autos.
Speed Limit Signs: Sometimes, a debate will arise over poorly placed or obscured speed limit signs. A dashcam may capture these signs to corroborate your claim.
Calibration and Placement of Speed Cameras: Your dashcam may provide evidence that disputes the placement or accuracy of a camera or radar gun.