Why earthing is required in star connection?

It is essentially a safety feature. The "neutral point" of the star (or Y) configuration in three-phase circuits would normally be at an electrical potential of 0 (zero) volts, but if the circuit is unbalanced the voltage at this point could rise considerably. If this point is earthed and any voltage appears at this point due to a circuit imbalance, an "earth leakage current" will begin to flow. This earth leakage current can be detected and used to trip a circuit-breaker to cut off the electrical supply for safety. Earth Leakage Circuit Breakers (ELCB) are commonly used in industry and around the home for reasons of safety.