In my almost 50 years of driving, I can think of only 1 time when a dash cam would have been valuable. That involved someone rear ending me on the freeway about 35 years ago and trying to say it was my fault. Luckily, there was just a small, almost imperceptible scratch on the bumper of my car so we just shined it on. His car had more damage but he knew he was in the wrong so he was happy just to forget the whole incident.
I actually have a dash cam somewhere that I bought about 5 years ago. I hooked it up in my car and took about a 30 mile trip with it. After I got home, I reviewed the video, it looked fine but I concluded that it just wasn't worth the effort so never used it again.
Having a dash cam is kind of like owning a gun for protection, the odds are that you will NEVER benefit from having it but it can come in handy in rare instances.