The Ming Dynasty had standardised on silver, yet China did not use silver coins. There is a good description of how silver was used in transactions in the introduction by Christopher Rea and Bruce Rusk to the classic text The Book of Swindles by Zhang Yingyu:
In the late Ming, [money] took two main forms: silver, which circulated unminted as ingots, and copper coins … Loose copper coins served for small, everyday purchases, and strings of up to one thousand (attached through holes in the center) for larger expenditures. Silver, because of its greater density, was preferable for major transactions or long-distance transport. Private silversmiths cast ingots of various shapes—the most common being a pinch-wasted oval with flat sides and flanges around the top—and varying degrees of purity. For a minor transaction, one might use a fragment of a gram or less, sometimes snipped off a larger ingot. … Although gold was valuable in the Ming, it never served as money, only as a precious material for making jewelry and other luxury items. … By the late sixteenth century, silver … was how many prices were denominated and how most taxes were paid and larger private payments were collected. After two parties settled on a price, hey had to weigh out the appropriate quantity, raking into account the proportion of silver in the alloy, which could vary from under 70 to nearly 100 percent,
It is easy to understand why a standardised coin like the real de a ocho ended up playing an important role in the Chinese economy and commerce.
The Book of Swindles is reviewed in the Asian Review of Books.