Question

why is commutator used in electric motors?

Answer 1

split rings acts as commutator

Answer 2

In a DC motor commutator is used to keep the torque from reversing every time the coil moves through the plane perpendicular to the magnetic field.  It is a split ring device.  So every time the COIL is at right angles to the magnetic field it reverses the current at that point.
The contact points are called  Brushes (nowdays these are spring loaded carbon contacts).