in my opinion both are currency. Bitcoin is the digital currency and other local currency that we can buy. Because bitcoin is the digital currency, we can only buy goods online. We can go around and do more things online.
Bitcoin as a digital currency is mainly used to consume your investment online and can be transnational transactions, and cash is limited, the national currency is not universal in every country.
Dollar and Euro can use on many countries, you can exchange to the national currency depend what country you are staying or traveling. Of course, if compare Bitcoin to the FIAT about the popularity, FIAT is still popular than Bitcoin a lot.