Hi Ravi,
In most organisations, the commercial invoice is created after the goods have left the Plant, and that is generally too late to create the Export Declaration (since related documentation might need to be sent with the goods). In that case, it is convenient to use a proforma to transfer the data to GTS.
But if the company procedures allow the commercial invoice to be created in time, then that can also act as the data transfer vehicle; then no need for a separate proforma.
Regards,
Dave