Once they know everything you buy or sell and can govern under what conditions you can use the currency, they have you by the short hairs. From J.B. Shurk at gatestoneinstitute.org:
[E]ven when you have followed every law to the letter and paid every fee to the cent, there is still no guarantee that government agents will not later invoke eminent domain laws to swipe what you own because they believe they can use your private property more fruitfully for the “public good” – and, since Kelo v. New London, even for someone else’s private good. So much for private ownership.
Competition, in theory, forces markets to naturally discard bad and expensive products, while keeping the prices of the best products low…. In practice, however, mature students of capitalism understand that entrepreneurs are never in search of markets for competition but rather conditions for maintaining monopoly.
The end result is that capitalists are always in pursuit of ways in which they may take advantage of laws and regulations, specialized knowledge, government contracts, or other exclusionary mechanisms to restrict potential competitors from ever entering the market.
The usefulness of money over traditional bartering for goods and services comes from its three chief functions providing (1) a unit of account, (2) a store of value, and (3) a medium of exchange.