Strictly speaking you don’t. But whatever currency you choose to trade in needs to have a couple characteristics:
- Lots of other countries want to trade in that same currency.
- There needs to be plenty of it to provide the needed liquidity.
Let’s say you are Brazil and you want to sell sugar to Saudi Arabia so you can buy gasoline from the United States. Saudi Arabia has dollars to spend. The US only accepts dollars. So the deal has to be done in dollars. And the more countries you trade with the harder it comes unless everyone is using the same currency. The US is the largest economy in the world and the number 1 importer and number 2 exporter. If you want to be player, you need to use dollars.
BRICS is mostly a development bank like the World Bank or IMF. The World Bank and IMF are controlled by the US (for the most part) and used to implement US/Western policy. BRICS is trying to counteract that and implement policies favorable to those nations.
So take one guess what currency BRICS uses to lend money. You guessed right, BRICS makes loans denominated in the the Yankee Dollar. Because that’s what everyone needs and there are enough dollars available for borrowing. Nobody wants loans denominated in rupees or rubles. China’s capital markets are closed, so the yuan is off the table. So that leaves dollars. Maybe Euros in a pinch.