Top 10 reasons to use a rangefinder
This type of camera slows you down. Traditionally, rangefinders aren't built for action sports. You'd be lucky to get a capture rate any faster than 3 frames per second, which I find refreshing. You have to take your time, slow down, and consider the photo. Getting that perfect moment because your camera can't take a sequence of images, which I believe lazy people do to get their safety shot. For me, I like taking my time when composing a photo. Less trigger-happy, more slow, methodical, and precise.
You can see outside of your framing lines. One thing about DSLR cameras that is an advantage is you can preview the depth of field. But the downside about DSLRs is what you see is all you get through the viewfinder. You could have framed a great shot, but just beyond your sight of view, you might have missed the perfect photo, and you would have never known. With a rangefinder, you're looking through a window left of the lens. Whereas with a DSLR, you are looking directly through the lens. The great thing about the rangefinder's window is the frame lines. The window you're looking through can view much wider than the lens you're using on the camera. Anything outside of the lens frame lines you're using can still be seen. Helping you compose, view, so you never miss a moment.
More accurate focus. Since a DSLR focuses through its single lens, it relies on software. A rangefinder uses two viewpoints to focus. Instantly improving its focusing system, making it more accurate when it comes to getting your focusing point.
No mirror blocking your view; you see the moment you captured. This is a major selling point for the rangefinder. When you press the shutter, you see the exact moment you captured clearly through the rangefinder. With a DSLR, when you press the shutter, the mirror flips up, temporarily blocking your view to expose the photo. Not having a mirror in a rangefinder is a minor advantage, but for me, it gives me peace of mind.
Smaller. Since there isn't a mirror flip space to compensate for, a rangefinder can be quite compact. For me personally, I like a small camera instead of carrying around a large brick. Less weight, less effort, less energy.
Focal-plane shutter helps me with handheld exposures. For me, I hate carrying around a tripod. I love the freedom a camera can give when there is no need to set up a tripod to get a sharp photo. When I try and do a long exposure with a DSLR, the mirror flip moves the camera ever so slightly, creating blur. Anything below 1/125 and it's blurry. But when I use my rangefinder, I can get sharp handheld photos at a shutter speed of 1/60 to 1/15 easily. The focal-plane shutter creates no movement in the camera, perfect for my style of photography.
Quieter. Again, it comes back to the DSLR mirror. It's the greatest feature and at the same time, it's the greatest flaw. If you want quiet, get a rangefinder; they make virtually no noise. If you want dead silence, then a mirrorless camera is the way to go. But you'll miss out on that amazing feeling a rangefinder can give you.
No lag looking through the rangefinder, just a simple window. One thing mirrorless cameras lack is instant. With a rangefinder, you're looking through a window independent of the lens and camera battery. No power or software needed. With a DSLR, it's the same, instant view. But with a mirrorless camera, you're at the mercy of the hardware and software; lag becomes an issue. Leica rangefinders are free of this problem. Another thing to note is that you're looking through a window left of the lens, there is no depth of field preview. You have to know your camera and what you are doing. For me, this is a plus; I like to know I took the photo, instead of the camera.
A less stereotypical professional-looking camera, more low-key. When people see a DSLR setup, they think professional or hobbyist. If you are a street or events photographer, people are going to notice you and your equipment. There is no discrete way of getting around it. When you don't want to be noticed, size matters. A small rangefinder says hobbyist or amateur. Perfect if you want people to ignore you or not take you seriously. Which can help you get that candid moment you might be after.
Not exactly what you see is what you get; for me, this is great. Since a rangefinder isn't looking through the lens, it's looking through a window slightly to the left of the lens. So you never quite get the same picture or view as the lens does. For me, this is great because it's made me a better photographer. You have to know your camera, know how everything works. Fluke photos might happen but they are few and far between. When you use a rangefinder, you have to know what you're doing. You feel more like a professional photographer than just a camera operator. This is why I choose to use my Leica rangefinder.