You only need it as long as you don't use it....

Terry Pratchett's book, Making Money, has a passage on gold that I found pretty insightful.
It is in Chapter 5 of the book, starting on page 136.

Moist grinned as the discussion wobbled back and forth. Whole new theories of money were growing here like mushrooms, in the dark and based on bullshit. But these were men who counted every half-farthing and slept at night with the cash box under their bed. They'd weight out flour and raisins and rainbow sprinkles with their eyes ferociously focused on the scale's pointer, because they were men who lived in the margins.

If he could get the idea of paper money past them then he was home and, if not dry, then at least merely Moist. "So you think these might catch on?" he said, during a lull. The consensus was, yes, they could, but should look "fancier," in the words of Natty Poleforth--

"You know, with more fancy lettering and similar." Moist agreed, and handed a note to every man, as a souvenir. It was worth it. "And if it all goes wahoonie-shaped," said Mr. Proust, "you've still got the gold, right? Locked up down there in the cellar?"

"Oh yes, you've got to have the gold," said Mr. Drayman. There was a general murmur of agreement, and Moist felt his spirits slump.

"But I thought we'd all agreed that you don't need the gold?" he said. In fact, they hadn't, but it was worth a try. "Ah, yes, but it's got to be there somewhere," said Mr. Drayman.

"It keeps banks honest," said Mr. Poleforth, in the tone of plonking certainty that is the hallmark of that most knowledgeable of beings, The Man In The Pub.

"But I thought you understood," said Moist. "You don't need the gold!"

"Right, sir, right," said Mr. Poleforth soothingly. "Just so as it's there."

"Er . . . do you happen to know why it has to be there?" said Moist.

"Keeps banks honest," said Mr. Poleforth, on the basis that truth is achieved by repetition.