Dashcam footage doesn't need to show the the exact impact to be useful. In the case of a side collision, as an example, the footage can be used to show if you were changing lanes, or stayed in your lane, in the moments leading up to the collision.I've always had the opinion that we need cameras on all four sides. Usually the dash cam only shows half the story as the end result is usually off to the side of the vehicle.
If you stayed in your lane and the other party claims you hit them, as in you were the one changing lanes, the dash cam footage would 100% validate your story over theirs.