Significance : Zynga dollars and Paypal dollars trade on the market at 0% interest. With the dollar trading at 0%, the dollar is officially worth the same as zynga dollars. The stuff you earn points for in WoW.
When the fed drops the interest rate of their loans (And remember, EVERY dollar is loaned) what they are doing is declaring it all worthless. We're at that stage now, and honestly - we've been "at that stage" for a long time.
But Currency is important, and only in a world without currency can be there any hope of barter. And you can't really barter your goods for EVERYTHING. I work in metal, but a doctor isn't going to be interested in my trade so that he can work on my hip. He would have to find someone who deals in .. say.. Coffee who needs an exhaust on his tractor. The doctor would get the coffee, the coffee grower would get an IOU for something, I would get my hip replaced - but not before giving a few hours with my welder at the coffee farm.
And that's a really easy trade.
So Currency is invented to be the stop gap there. And what's great - We HAVE this ability - today.
Before the federal reserve came about, different regions of the country would print their own currency based on whatever collateral they contained within. The Philadelphia silver note, for instance, was based on the silver in the basement in the bank that printed it. Ansonia CT (Where I lived a few months ago) had it's own printing press. And beneath the press - a gold vault.
So what kind of things would your area hold to print it's local money ? What kind of stuff would they give and take at the border of the next town or area to trade for what their currency represents ?