It is because of the deficit we run finance ministry has to thread carefully with respect to spending. For example let's say we cancel out the deficit or run a surplus. Then RBI doesn't need to dance too much to defend INR. In fact they don't have to do anything in that case. Now inflation will be under control. GoI can easily borrow lot of money from the market and invest in critical infrastructure. Now all the FDI and remittances we get are massive infusion of cash into our economy which doesn't get cancelled out since we are not running a deficit. That is how Chinese are able to print money and invest vast amounts on infrastructure whether needed or not. They run massive surpluses and they can defend their currency anytime.
And this will have an effect on GoI revenue as long as they are investing borrowed money into productive sectors.
Your mixing up Trade Deficit of India with the Fiscal deficit of GoI. Though there is a complex relation between the two, both are not directly proportional. As you can see from the charts.
Current Account Deficit
Fiscal Deficit
Yes the government will greatly benefit from a trade surplus which would result in a net inflow of wealth. But government borrowing mainly depends on the outstanding interest payments & the debt to gdp ratio which the government feels comfortable to maintain.
Now considering government debt financing, of both external debt & internal debt. The external debt is only a fraction of the total debt of GoI.
If you see the charts the government external borrowing doesn't depend on the current account deficit but is proportional to the GDP of India.
The government has tried to maintain External Debt to GDP ratio at around 20% for the last 2 decades.
Even though the Forex to GDP ratio has improved considerably.
So a trade surplus with not lead to a sudden borrowing spree for the GoI. The borrowing will increase with the increase in GDP.