Legal tender
The point is that other kinds of money have been used. Legal tender differs from commodity money and representative money. Commodity money is based on a "good", often a precious metal such as gold or silver, which is desired or has other uses. Representative money is a claim on the commodity rather than the actual good. In that case, the gold stays in a bank, and its ownership changes from time to time as the money is used. Why might a good be used at all? One reason is that governments cannot just make it. It is said that this stops inflation because, with fiat money, governments can just print more.