Ask Lemmygrad

The fact that the dollar and the euro are the world's reserve currencies allows both America and the EU to do 2 crucial things.

Firstly, whenever a country prints money (they don't exactly, the money supply increases when banks give out loans. Countries can loosen lending requirements/costs), it acts as a tax on everyone holding the currency, because the value of products purchasable by that currency is spread over more currency.

Since everyone holds dollars and euros, American and European banks are able to tax the world. The same applies to their treasuries, which increase the money supply by running deficits (financed by borrowing from their central banks).

Secondly, these countries can sanction individuals, organisations and countries by cutting them off from exchange markets (as described by soviet entropy), and by cutting them off from the swift banking system (which is tech to transfer financial messages/transactions internationally).

These are the 2 fundamental tools of imperialism, and practically define it. The role of currency in empire is so important in fact that the first currencies were invented by ancient empires to tax subjects.

BRICS becomes an obstacle to the American and European empires by offering financial alternatives, and allowing countries to, if they choose so, to free themselves from imperial currencies. But this capability hasn't been fully developed yet.