Starter are basically used to reduce the starting current.  
I = ( V - Eb ) / R.
Where I is the field winding current,
V the voltage and Eb - Back up emf and R the winding resistance.
During starting Eb = 0, and R - a very small resistance.  
so, as the high current passes through the winding, there is
chance for winding to burn out.  When the motor attains the
normal speed it generate the back up emf, enough to reduce
the winding current.  
To avoid this we need to reduce the voltage(V) or increase
the winding resistance.  Since the reduction of supply
voltage is difficult to attain, a starter is used, which
will push high resistance during starting and  gradually
reduces and fully removed from the circuit when the motor
attains the full speed.