Why have commodities saved as money in the past?

Commerce has always involved people trading items of worth - Mr. Smith has an object Mr. Jones wants, and vice versa. And a long time ago, both parties had to have actual objects - perhaps Mr. Smith has fish and Mr. Jones has beer. The problem is, if they're in Gloucester or Astoria Mr. Jones is going to have more fish on his hands than he knows what to do with. His only way out is to take the fish to a town where they have none and trade it for beer ingredients.