Just as we sometimes have trouble traveling with money to other countries because of exchange rates and who will accept our currency, this could happen as you travel from state to state. The Arkansas dollar might be worth less today than the Texas dollar as you travel west so you get more for your money in Arkansas if you are carrying Texas money but on the way back there is flooding in Texas so the value of their dollar has dropped so now the money you are carrying is worth less than if you had Arkansas money. This is just an example but those are the kind of problems that would arise if each state had their own money.
