There is simply nothing faster than "not" auto-focusing at all. Snap focus is faster than any auto-focus system. It would be a great feature to have.
The reason you might use it for example is: say you have focused on something in the distance (20ft), and something comes close in the foreground at 3ft suddenly that I want to be in focus; if my snap focus distance is set to 3ft, I can jam on the shutter get whatever is the foreground in focus.
What if the closer subject is at 10 feet, 5 feet, 15 feet etc, etc? Maybe I'm missing something but it seems like it's usefulness is quite limited. I don't do street photography so maybe I'm missing something. According to reviews the GR III AF isn't great, certainly not in RX100V/VI class, so maybe for that camera it could be useful.