If a country goes into war, IMO its currency would actually depreciate (not go up), because it would incur unplanned expenses, which would increase its spending, widening its deficits and weakening its economy.
In such a situation, holding a depreciating currency, would hurt, as we would have spent more to buy it, than its current worth.
Holding a currency as reserve makes sense because we transact more in that currency and we anticipate it to appreciate in value because of the demand for that currency.
If, USD appreciates, having an appreciating currency in reserve would only benefit us, as we could sell them to fund our imports and stabilize our currency (INR). Moreover exporters will actually benefit from it, it is the importers that will feel the pinch if USD goes up.
For a country (India) which is a net importer, we pay more USD than we receive, so holding USD as reserve helps in softening the fall of INR, and brings stability.
IMO, currency appreciation/depreciation is mostly related to its demand and supply.
When everyone pays in USD, the demand for USD increases driving up its value, to curb this, we need to increase the supply of USD, that is where holding such reserves help, now RBI would sell USD, increasing USDs supply and removing some INR from circulation, this to some extent prevents INR from depreciating steeply.
We just want rupee to depreciate at a slower pace by holding more foreign currency reserves. A stable currency brings stability to our economy.