Anything is only worth as much as someone else wants it. Everything comes down to supply and demand. Currency is just a medium so you don't have to trade fish for lumber in order to trade the lumber for some bacon. Also, pretty convenient to be paid in currency instead of fish, lumber, or bacon. You can use it to directly buy/get what you want.

If no wants to buy or sell the currency then it's a pretty worthless currency. That's why you can't just crate your own currency and buy things with Bob's Bucks.