…. because you will never miss what is going on outside of the frame.
Joke aside, and in times of inevitable comparisons of the new Leica M240 vs M9 vs M8 vs D800 vs film vs Toyota vs Ferrari vs my father is stronger than yours, I am convinced that the one feature that make rangefinders especially great for street photography is the frame lines system. Indeed, it allows you to better anticipate the "decisive moment" by visualizing, directly in the viewfinder, the elements that are about to enter your scene. No need to scan around with your camera, just position your frame and wait for the moons to align.
So sorry, this in not another long "10 reasons why rangefinders are better for street photography", because I guess that #1 simply trumps all the other ones. (click on picture for high res)
Rosario – Leica M9 with 35mm Lux Asph at F1.4, 1/1500, ISO160