Essential, but can be used against you as my daughter has found out. The somewhat unclear (due to window reflections) dashcam footage in her car shows a tractor sweeping around a bend and hitting her. As support, seperate phone pix of the road shows the amount of road he took up, leaving 9 ft unused on his n/s. Guess what? Her insurers say the ( somewhat unclear) dashcam footage shows that both my daughter and the tractor were on their own sides of the road and that he 'seemed' to stop a fraction of a second before her, so it's 50/50 and no discussion. Witnesses, passers by and all the evidence say the contrary but it means nothing because the dashcam 'seems' to show otherwise in the insurers eyes.
So, think about what the dashcam shows and only use it if it's undisputable.