Money is a polytheistic religion.
Stay with me for a second.
The only reason money (fiat money, that is) has any value is because people believe it has value (at least since we stopped making coins out of precious metals, anyway).
A dollar bill has no intrinsic value; its value is based solely on how many people believe it has value, and how much value they believe it has. They have faith that when they take that dollar bill in exchange for something, they, in-turn, can give it to someone else for something else. Every transaction in which real goods are exchanged for fiat currency is a small leap of faith.
The worth of the currency is proportional to the amount of that currency in existence, and the collective amount of faith that the people of the world have in that currency's worth.
Example: Which would you rather have? 100 British pounds or 1893.54 Mexican Pesos (equivalent amounts, based upon today's exchange rates). I'm going to go out on a limb and suggest that most people would rather have British pounds, since their economy has a better reputation and their government is much more stable. It's easier to put your faith in the Pound than the Peso.
Why do we do things this way? It's much easier to trade 'value' around using currency than it is to just trade goods and services. I don't have to work for the manager of the Burger King for an hour, or trade him a bag of tomatoes from my backyard garden if I want to get hold of a Whopper with extra ketchup. Currency makes trade much faster and easier, and simplifies the process of paying people for services like labor and expertise.