Yeah I don't get the box squats. You can use mirrors for depth most places or take a video. I think the box squats would put more stress on the spine on impact.
They don't if you do them properly. And they're not used to measure depth. It's to help you get better at getting out of the hole of a PL squat.
I have seen the video Gymrat posted before (thank you for sharing). I don't go that extreme on the stance enough to upset chaos, but I do them wide, and try to keep my shins upright.
They are not meant to be a substitute for squats, they are just another tool in the tool chest to aid in getting stronger. I use them because I believe they are the number one factor in making me faster as I get older. YMMV, but it works for me.