Cryptocurrencies have no intrinsic value. They’re just numbers stored on a system somewhere, much like the way modern digital banking treats national currencies. If A believes B will accept USD, then A will accept USD for whatever they want to trade, too. B will only accept USD if s/he thinks C will also take USD, and from this collective faith, the value of USD arises.Cryptocurrencies are similar.
They are only worth as much as everyone deems them to be worth. One advantage for national fiat currencies is that a central authority issues and regulates them, and one can generally trust the government in this regard. Furthermore, governments require that taxes be paid, and they will usually only accept their own currency for this. Hence, to be a citizen of a country, one must deal with the national currency. This is a good basis to simply use the same currency for every transaction within the country.