Yes, most commerce in any particular national currency is domestic. But the amount of capital reserve a treasury or central bank needs in foreign currencies sca

Yes, most commerce in any particular national currency is domestic. But the amount of capital reserve a treasury or central bank needs in foreign currencies scales with the percentage of their commerce that is import and export. If less than 1% of trade is with external economies, that's not too big of a deal. If 10% is, that's a huge amount of capital that has to be taken out of productive use and put towards purchasing foreign currency to stick in a vault. It doesn't have to he the majority, it just has to be a not insignificant amount to see the problem. And then any significant changes create crises like this, where a government has to scramble to purchase foreign currencies or change interest rates to adjust their currency supply, these all have impacts on the domestic economy such as inflation, deflation, unemployment etc.