Fiat Money became so successful because it rewarded entrepreneurs with Newly Created Money. Newly created money has yet to create inflation, so the holder of newly created money has an unfair advantage to buy everything cheaply before prices of everything goes up. If those people with unfair advantage are entrepreneurs then the economy grows, as they increase productivity, make everything cheap etc.

The commercial bankers decided if their balance sheet could accommodate the risk of newly creating money for such entrepreneurs and they took a small fee (interest) to cover the risk of default.

In case you didn’t know already bankers create new money when they issue new loans. Issuance is regulated by cash reserve ratio and other such regulations.