Those who have money do not have to put them all inside of their country’s banks. It is possible to deposit some of them in overseas banks in a practice known as offshore banking. It is a legal maneuver that people can use to spread their wealth and lower their risk. In case there is a geo-political issue in their home country, their money will still be safe and sound elsewhere.
A lot of individuals use this method to diversify their currency portfolio. If one currency plummets because of economic mismanagement, then this can be offset by their funds in other currencies. There tends to be higher interest and lower taxes for deposits of this kind — at least, you can choose to go where the situation is favorable instead of being bound to the conditions in one area. Privacy and security are also improved.