I second this. It could save you thousands of dollars should something happen.

I got one after a person who hit my car after pulling into my lane and thus into the side of my car, tried to tell their insurance (despite admitting fault at the scene) that I had hit their car while it was parked on the side of the road. Luckily I took photos of the scene immediately after it happened which clearly showed that was not the case but a dash cam would have likely been even better.