64 bits can be tight if you have to set aside, say, 7 digits for decimal places in order to perform a lot of divisions/multiplications by small numbers, like ex

64 bits can be tight if you have to set aside, say, 7 digits for decimal places in order to perform a lot of divisions/multiplications by small numbers, like exchange rates, without rounding errors adding up. But if you sacrifice 7 places you can no longer represent the market cap of Toyota Motor at the Tokyo stock exchange (24 trillion Yen) or the US GDP (~19 trillion USD). So if you don't have a data type that can represent all of these quantities as well as the results of any calculations up to the required precision then your code will become a lot more complex and error prone.