Why money exists
Back in the day, people used to trade stuff if they wanted something. Let's say some guy named Tom had access to an apple tree. Tom wanted a bushel of wheat so he traded a few apples with his neighbor who had access to wheat. After the trade, a little piglet the neighbor had caught Toms eye. So he asked to trade all the apples he had for it. The neighbor explained that although he was very interested in the favorable deal, he couldn't possibly eat all those apples before they expire. Herein lies the necessity for a solid medium of exchange. Even if Tom had an unlimited supply of apples, because of their short lifespan and copious supply, his bartering options were reduced to small transactions. If he wanted to start buying more valuable items, he'd need to start trading his apples for an item that lasts much longer. This is where precious metals come into play.
The Roman Empire
The romans were one of the first to really take this concept and run with it. When you consider the benefits of having a solid medium of exchange like gold coins that are universally accepted, it's no wonder the civilization was so successful. If people (or machines) have access to a reliable medium of exchange, the types of transactions they make can vary in size and the services they provide can have a wide range of specialization. With the inclusion of copper and silver as less valuable storages of wealth, Tom was able to sell his apples throughout the week receiving copper coins and later trading those coins for the piglet. The neighbor accepted the copper because he knew it would retain its value over time until he was ready to exchange it for something else. Only so much metal exists on the planet and harvesting it took a lot of time, effort, and skill. We can anticipate how much gold, silver, and copper will be introduced into the market every year based on the information and techniques used the years before. The system was a reliable one, and it served us for centuries... until it didn’t.