why we are earthing the nutral wire in the sub station even we have aearthing in home?

Answer 1

Since there is a transformer near my home, there is no galvanic connection between my home and the substation for any leakage current to flow. There must be an earth connection on 'my' side of the transformer AND on the substation side of the transformer for electrical safety, in case of a fault, or an open neutral (return) wire.

Even if there *were* a galvanic connection, the resistance of the grounding wire would be very great, and if some fault current were to flow in that wire, all the way to the substation earth connection, the voltage across the length of wire might become dangerous, itself.

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Answer 2

The consequences of an ungrounded system are so devestating, it is useful to provide an extra level of protection.