- "Everything is worth what its purchaser will pay for it."
– Publilius Syrus
Currency adds +1 trade routes per city and allows you to construct the Market and Grocer (with Guilds) city improvements. It also enables you to build Wealth in cities and to trade gold with other civs.
Currency is an advance greatly important to early financial gain. The trade route will often be the first one players get in cities (adding to commerce), and Markets add to both cash and happiness - ideal for Holy Cities.
As cities grew, their internal economies became more complicated. People became specialists, some primarily producing grain, some pottery, some bricks, etc. A system of barter developed, so that one individual's wares and services could be exchanged for those of another. Gold and silver were widely used in such transactions, but the pieces needed to be weighed and tested for purity each time they changed hands.
In 600 BC, the Lydians found a way around this problem. They began the practice of shaping electrum, an alloy of gold and silver, into pieces of predetermined weight and purity, stamped with an official symbol. This idea caught on, and within 50 years all the important trading centers of the world had adopted similar practices. The widespread use of currency created universal standards of value, allowing people from various societies to conduct business almost anywhere without being forced to bring along bulky goods for trade and barter.