✔ 最佳答案
This isn't a solid answer, but more of a theory. People have talked about the U.S. dropping the penny since it costs more than a penny to make a penny which is inefficient.
Way way back when precious metal coins were used, they valued those coins, for one reason or another so it was just like trading, then the coins started to just represent value, as in, "I worked for this product, here's proof." and now we have paper currency, which is a cheaper way of representing. And now we are moving to credit cards and "virtual" currency which does the same thing. You do a job for someone then they give you "proof" that you have done work and so deserve a certain product.
This is not necessarily backed up by anything, but just my thoughts on it.