Prior to the early modern period, when silver was more readily available thanks to mines in the Andes Mountains in South America and in Japan, there was a general scarcity in silver and gold. Silver and gold that existed was often converted into dishes, silverware, jugs, or objects of worship—items that would show off power and status.
Instead, people would rely on bartering, the trade of goods as a form of payment instead of coinage or metals. This is particularly true of medieval economies, which functioned based off of a series of exchanges between social classes. Instead of paying for goods and services with silver or gold, one might pay for a large good or service with livestock or hard-to-find items or a small good with agricultural yields or homemade items.
This started to change with the Holy Roman Empire under Emperor Charlemagne, who introduced the silver penny. This dominated European economies from the eighth century onward, until gold became more readily available and when silver mines in other locations were mined by European merchants and joint-stock companies. In fact, Charlemagne's silver pennies remained the standard for European currency until 1972, when it was replaced with new units of measurement.
Silver was adopted because it could be used as a standard currency; despite who traded it, the weight was the same, and it helped connect growing economies. Furthermore, it was more regulated than the bartering economy, which was much more subjective based on who was buying and who was selling.