There are countries in the world where the main currency is the USD. That is, the national currency exists nominally. Using cryptocurrency as the main currency is unrealistic so far. Volatility is sometimes too high.
Well, not all cryptocurrencies are subject to great volatility, for example, the price of USDT changes but not very much. Such a cryptocurrency can be used in such countries.
USDT is a stable coin. The price of USDT depends not from market conditions, it depends from the price of USD.