Explain the function of commutator in DC machine?

The commutator in a DC Motor directs the applied DC voltage/current to the specific coil/coils such that the resultant space flux in the armature is displaced ideally 90deg with respect to the field flux, even though individual coils are rotating along with the armature, thereby ensuring the development of maximum torque. In simple words, if one coil in the armature is energized and forward torque is produced, the armature with the coil rotates. Thus, the coil position changes and it may not now be able to produce forward torque (since its position with respect to the field flux has changed) to keep the armature running. Thus, the specific coil is disconnected and another suitable coil is re-connected, all through the commutator, such that the motor is kept in rotation along with required torque generation.