why neutral is grounded in power transformers?

To avoid damage to the transformer and other electrical installations connected to the transformer, from voltage transients that would result from line to ground faults in an ungrounded system.

You need a neutral for the reasons; picking of earth fault relays for ground fault to complete return path of earth fault and to give trip command to circuit breaker through which fault current was passing, other reason is to avoid floating neutral affecting voltages to save transformer as well as loads connected to it..

In case of ungrounded power systems we have overvoltages at phases which aren't exposed by fault, so insulation of this conductors could be seriously damaged or in best case it could become older in shorter time than it is provided by design what is the main reason for grounding of power systems