Donald,

Rangefinders allow the camera to operate without a mirror and in the case of the Mamiya without a curtain shutter. This makes for smooth, silent, motion-free operation while making a picture. The Mamiya can be used a few feet from someone in a quiet room without the person hearing the shutter. You can hand-hold it to a much slower shutter speed than you can an SLR camera. Also, not having a mirror to move and slap out out of the way at the time the picture is made means a much lighter camera. The Mamiya 7ii is one of the lightest cameras of any format, and absolutely the lightest and most compact for a rollfilm format. Leicas, as you are probably more familiar with, are also remarkably light and small. And very rugged for their weight.