Main Roll of Back emf in DC motor

(1) It protects armature conductor from short circuit.At starting position of motor.It cannot rotates immediately due to inertia at that time N=0, and Eb=a. Thus maximum current is flow from the armature.
(2) It helps to produce required amount of torque.When motor is over loaded it's speed decreases and then Eb also decreases but Ia increase Eb=Z
So the motor develops more torque to over come additional load.Similarly when the motor is under loaded,it is speed increases and Eb also increases but Ia decrease so the motor develop less torque to compensates the respective load.In every energy conversion their must be an opposing agent in this case Eb.Opposes the source voltage V.d So that the source V has to supply current against the Eb and the cause to rotate the armature and hence electrical energy is converts in mechanical energy.