consider a 3phase induction motor,it is a  selfstarting
device.Eventhough it is a selfstarting one we are connecting
this device to starter because if we can not connect this
motor to stator because of sudden flow of currents in the
rotor windings some damage may takes place to the machine.so
we can start up the motor with starter means we can limit
the currents to certain value upto which the rotor gets
accelerated.When ever rotor attained its rated speed then
there is no problem of current flow.