One additional wrinkle in all of this is that money was a very straightforward way to fund conquest. Soldiers are paid in coin minted by the army they work for

One additional wrinkle in all of this is that money was a very straightforward way to fund conquest. Soldiers are paid in coin minted by the army they work for and the conquered are required to pay taxes in the same coin. Conquered villages are thus required to monetize in the specific coin the conquerers made and the conquering soldiers are able to sell their coin for whatever service they need at whatever rate they set. At the risk of sounding incredibly pithy, the primary purpose of money over barter is to aid the tracking and collection of taxes.