By being so organized, the bank is performing the economic function of maturity transformation. A deposit, which is potentially demandable at any instance, has effectively been lent out through financial intermediation in the form of a longer-term bank loan that is not making repayments at each instance. And yet… the beauty of the arrangement is that most of the time, this works out.
The day my health expenses arise and I take out money from the bank, my neighbor and others have likely received monthly paycheques, a part of which remain deposited in the bank, or that same day some loan repayments have been made, extending the savings pool of the bank and allowing it to meet my deposit withdrawals.