Couplings mechanically and electrically isolate the encoder from the motor. This separation helps avoid encoder signal errors such as miscounts or extra counts by reducing noise in the encoder. In addition, electrical isolation prevents damage caused by current from the motor.
Couplings also allow the encoder to compensate for shaft misalignment. Cutouts on the coupling allow flex to occur in the coupling rather than in the motor shaft. The runout from the shaft is absorbed by the coupling to prevent physical damage to the encoder.