Modern society is only possible because of global trade networks. Global trade networks would never work without currency. If a person spends all their day fabricating metal sheets they need a way to buy bread to feed their family. Otherwise we'd all be back to farming our lives away.
Even in a non-capitalist economy, currency would still make trading easier. It simply might not be exchanging hands between private individuals but between governments.
Capitalism is not trade in and of itself. It has to do with who owns the means of production.