I was standing there, able to convert my ratty, purposeless piece of paper into food because of a magical belief system. I believed in the efficacy of money, as did the guy at the cash register; we believed in money because everyone else believed in money. Our collective faith in the floating
The practical effect of this: my willingness to sit in an office all day typing things for other people so they would give me magical paper that I could convert into things I needed to live (and things I didn't necessarily need to live).
That, of course, is just one view of money. Another view is that money is simply a shorthand, a way to translate values, a way to allow complicated transactions to be measured by one all-purpose measure of value -- the only way to conduct business in a complicated economy.