Originally Posted by pluton
It's not just the finder.
Leica, in their M lenses, is the only camera/lens company that I know of who's lens markings support zone- or scale-focusing in any meaningful way. The ability to accurately pre-set focus is critical to watching the scene and not a data display.
I don't know of any manual focus lens for any of the major camera brands that doesn't have a depth of field scale engraved on the lens. Leica lenses are not unique in that regard. You rarely see depth of field scales on autofocus lenses, even Leica autofocus lenses like the TL and SL series.