The key advantage of a TLR over a classic rangefinder is that the viewing lens shows a real image on a ground glass screen, so you can both compose and focus directly. A rangefinder’s finder is mainly for framing, and focus is done through a separate rangefinder mechanism; framing can also suffer from parallax.

Historically, that made TLRs a very practical consumer design: they gave photographers a bright waist-level image for composing and focusing without the complexity of an SLR. They were especially useful before modern viewfinders and rangefinders became easier to use.

Other practical benefits:

• very quiet operation, since there’s no flipping mirror
• easy waist-level shooting, including low-angle shots
• discreet for street photography because you aren’t holding the camera to your eye
• the ground-glass image can encourage different, more deliberate composition

A major modern reason people still use TLRs is that many of them shoot 120 medium-format film, which offers a larger image area than 35mm.

So in simple terms: a TLR sits between a rangefinder and an SLR—quieter and more discreet than an SLR, but with direct ground-glass composing/focusing that a rangefinder doesn’t provide.