Yes, it’s a rear-view mirror for divers.
And yes, I know exactly what you’re thinking.
Yet, there I was, back at 6m after a very nice dive on the wreck of a World War One U-boat and watching the contents of my dil cylinder dropping faster than my remaining decompression time. Divers pointing and making upward bubbling gestures reassured me that it wasn’t too serious, but it would have been nice to see for myself.
Then there was the time I was leading three sets of buddy-pairs through the wreck of the Rosalie Moller in the Red Sea, and spent most of the dive looking back between my knees or turning round to check where they were.
Or every time I lead down a line and want to make sure that my buddy is following me, especially when there’s a bit of a run because we’re early for slack.
Or the time I was videoing pike and managed to foul my first stage on the line linking attractions in the lake. Backing up was all it took, but being able to see the problem would have been nice.
I could go on, but by now you’ve got the idea, and you’ll be able to think up examples of your own.