The mirror section slides down and gives you a convex mirror that shows you the back bumper (rear fender) and the about-6"-of-space behind it - this allows you back right up to the vehicle behind without touching.
It's a bit of a moot point with the spare wheel being there but I've been driving since 1986 so you get used to checking your own car's reflection in the surrounding vehicles or shop windows.
Or get the Mrs to get out and indicate the distance to you in the rear-view using her hands to show how much space is behind you, like the opposite of a fisherman's tale