It has good properties to make jewelry and shiny things that didnt rust or loose quality over time and to be used as a conductor in modern electronics, not so much to make weapons, armor, tools, buildings. Imagine the dude trading all his gold to hoard iron because it made better swords lol. It had enough properties to have value, and the best properties at the time to be used as money. The conductivity properties became more useful later on in its lifespan.