I have one that records over the SD card and saves when it senses a crash or I can press a button to record 30 seconds of video. Had a kid back out a drive and it me. His insurance claimed I should have reacted in time to stop. Sent them the video and they accepted responsibility. Mine turns on when the car is started and off when the ignition is turned off. I’ll never drive without one again and should probably get one for the back view as well.
I have front dash cams for all my cars, but would like to add back ones too. I basically made the decision to do it when my daughter got her permit last year, so no one could claim any mistakes on her part, that weren't true.