Why is star delta preferred with induction motor?

to control the starting speed of motor use star-delta configration

Star delta starter is used to reduce the starting current drawn by the motor. Start the motor in STAR and then, when the motor has gained sufficient speed, change its connections to DELTA to allow the motor to run at its full speed and torque from then on. The motor starts as star connected, ie, voltage reduced by root(3), current reduced by root(3) and power reduced by 3. After the motor starts, it is switched to delta connection and runs normally.