Monopoly On Power
Thousands of years ago, a small group of people - who somehow happen to control the food, weapons, and knowledge in a society existing somewhere between tribe and civilization - decide that they desire to acquire even more power, even more dominance, even more control. This group's plan is to replicate its power, its dominance, its control - its "wealth" - through inventing a new resource that represents a rather abstract concept: "value."
They call this new resource "money." Now this small group of people not only has food, weapons, and knowledge, but it also has money - as much money as it has food, weapons, and knowledge. These conspirators just doubled their networth, and their control, through the invention of currency.
Thousands of years later, in the year 2014, currency isn't backed by anything but so-called banks who, by virtue of a globally orchestrated federal reserve system, have the ability to create as much new currency, as much new money, as much new control/dominance/power - as they'd like.
The best possible explanation for the need of currency is the desire for a monopoly on power.