I just realized it lately, yeah why we have to use paper as money if our ancestors successfully live through trading "barter", exchanging of goods to goods/services vice versa.
People think about convenience in everything so if some kind of medium can act as valuable thing to exchange between the goods that why the paper money and coins were introduced but the fiat money system comes very lately just before two centuries ago in my knowledge at that time only the banks take over the currencies from governments.