Some applications require very tight precision between all components. In these applications, even 1° of movement between the input and the output when a coupling is engaged can be a problem. This is true in many robotic applications. Sometimes the design engineers will order clutches or brakes with zero backlash but then key them to the shafts so although the clutch or brake will have zero backlash there's still minimal movement occurring between the hub or rotor in the shaft.
