Question

Why starter is necessary for a DC Motor

Answer 1

at the time of starting a DC Motor  , the Back EMF developed (E=K*PHI*N) is zero as it's speed is zero. So a very high current flows (which is limited by resistance only but resistance itself is very low usually 1-2 ohms) through the armature of the machine which can destroy it completely.
So to protect the machine from such large currents , starters which can be simple resistors or the sophisticated 3 point or 4 point starters may be employed. 
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Answer 2

The starter of DC motor is a variable resistance in series with the armature circuit DC motor to reduce high starting current