The rotor of the electric motor needs a torque so that it is able to start its rotation and this moment of torque is usually promoted by magnetic forces generated between the rotor and the stator magnetic poles. Forces acting on the attraction or repulsion, generated between stator and rotor, are able to pull or push the movable poles of the rotor, and this produces torques, which cause the rotor to rotate faster, until the frictions or charges attached to the axis can reduce the resulting torque to zero.
An engine can not function if it is made with permanent magnets only and this is simple to note since it will not present the initial torque to start the movement and if they are already in their equilibrium positions, they will only hesitate around that position if they receive a stimulus external.
After that moment, the rotor starts to turn at a constant angular speed and the rotor in the same way as the motor stator, must be magnetic, since it is these forces between poles that are able to give the necessary torque to make the rotor rotate. In the same way, even if permanent magnets are used frequently, especially on smaller motors, remembering that at least some of the magnets on a motor need to be electromagnets.