IIRC the original "banks" were basically just security guard teams that would hide and protect your gold and other trade valuables for you while you used currency on your travels to represent your gold etc, much easier than hauling all your precious metals around with you and the currency was to show how much you had in your vault, then when deals were done the gold etc could be physically exchanged for whatever, something along those lines but banks were just groups of brutes for hire to guard your shit, even then there was still always the risk that the group you hired would steal from you or have you disappeared before taking everything for themselves, having other people control your wealth has always been a high risk luxury