Pairs of people | companies | countries can directly trade in goods and services by some agreed ratio if that can be satisfied (eg: Australian Iron Ore + Rare Earth Concentrates in exchange for Chinese electronics is an ongoing tade at some agreed ratio)

A currency is abstraction for trade in wider circles where (the common case) trade between two parties isn't sufficient ( typically parties supply | demand a basket of various resources, various energies, various intermediate and finished goods ) and for trade to take place the guarantors must have faith in the agreed abstraction for transfer of credit; additional mechanism that verify transactions are an added bonus.

Both now and through history many currencies have been used and are being used.

I've given examples of US dollars, the Pound Sterling, in the context of this sub thread bitcoin is an obvious example of a currency that has been used to facilitate trade; less obvious but important examples of currency are the word of a trusted person of faith (as used historically along the original Silk Road and today throughout Hawala and similar mechnaisms).

What strikes me as odd about your question is the implicit assumption that I favour one singular currency over all others.