What makes good currency is that it is salable or easily sold. With the technology of smelting metals, early pre-Christian civilizations could make highly salable coins that were also extremely portable.
The preferred type of coin was gold.
When transportation and communications boomed from the 18th to the 20th centuries, nonphysical forms of payment like paper bills and checks became justified. The way they convinced people to trust these pieces of paper was by backing them with gold.
Britain led the way with this standard to back bills with gold, known as the “gold standard.”