It’s one of those freshnel lenses you glue to your rear window to act as a fisheye lens. It’s actually surprisingly good. I can see the ground about 1.5m (5ft) behind me. I wish it would reach my rear bumper as that’s what I’d like to see but this helps!
Also, I took the picture with a poor man’s smartphone - my apologies.
It makes looking out wider, bit that means looking in would be more focused. Careful it doesn’t light your car on fire in the sunshine.
I actually thought of the same thing and wondered if that’s the reason I dont see these more often.