Currency is a natural evolution of commerce. Direct barter only works if the person selling what you need wants something you have.

Say you want to buy flowers. If the florist wants shoes and you only have bread or hammers to spare, then tough luck.

Any large society cannot function with such a clunky way to exchange goods/services. Currency is merely a proxy that allows both sides to trade their goods using a tool they both value similarly. Hell, some civilisations used giant boulders as currency... it's hardly a new concept.