A country that controls a currency that is used by many people outside its own borders is analogous to a cryptocurrency with a "proof of stake" consensus algorithm in which holders of sovereign bonds are rewarded with "interest". The central bank may periodically make policy pronouncements which may be analogous to various soft and hard forks (change of interest rate -> change of mining reward) which may impact the valuation relative to other currencies. Certain countries/currencies have a very long track record of not having very many "zero day vulnerabilities" and are therefore highly trusted and a first choice whenever there is a "flight to safety" (when panic strikes and there is demand for "safe" investments).