i think that, in that period, banks were mainly used if somebody was moving a great distance from where they lived.

they would contact a banking institution (like the templars) who would take all their available wealth and give them a letter or some other token that they could 'cash' when they got to their final destination with the banking institution there.

the history of banking goes back further than the medieval period with evidence of grain loans and clay records of agricultural goods (though this was likely more of a 'grain census' imo.