Rangefinders are mostly used for photographing people. People tend to think that the larger camera you have, the more proffesional it is. Rangefinders are typically small. atleast the ones for 135-format, so people are more relaxed in front of the camera and tend to pose less. Beeing photographed is not a natural thing. Small rangefinders typically have the viewfinder placed far left on the camera. This means you're not covering your face behind a large bulky SLR while you're shooting. In other words, the person getting photographed is not staring into a machine but at the photographer.
Another story is that of Leica. Rangefinders would probably not be used a lot if it wasn't for Leica. Many famous photographs have been shot with Leica such as the portrait of Che Guevara, the d-day invasion of normandy and the little girl running away from napalm in Vietnam. Photographers such as Henri Cartier Bresson, Elliot Erwitt, Ralph Gibson, Sebastiao Salgado, Reni Burri, Anton Corbijn, Robert Capa and many others all used/uses Leica.