A base currency is not strictly required if you only have transactions in a single currency or want reports in the multi-commodity view only.
A base currency however is useful if you want reports in a single currency, i.e. the currency of the country that you're residing in. This makes many reports a lot easier to interpret as the values presented will be one that you're most familiar with.
Defining a base currency
You define a base curreny in Settings > Configurations:
Just type any of the currency in text form, i.e. $, RM, etc. and your configuration will be automatically saved.