As stated some where above, the currency, whatever form, is a trade medium. The worth of some item or service becomes equated with a certain amount of the medium and the medium gets traded. It's very convenient and saves transportation cost, preservation ( of food stuff ) and time, among other things. We could live without it but I'd rather go to market with a few pieces of paper or a couple of slugs of silver or gold than carry potatoes ( I had to check with Dan Quayle on this spelling and there IS an "E" in it ) or hog bellies around in my pockets.