The most valuable service banks provide is not just "matchmaking" lenders with borrowers (an exchange of some sort can do that much more cheaply) or even spread

The most valuable service banks provide is not just "matchmaking" lenders with borrowers (an exchange of some sort can do that much more cheaply) or even spreading credit risk across a pool of borrowers (again there are other ways to do that which are cheaper), but maturity transformation: depositors can lend money to the bank which is repayable on demand, but the banks' borrowers are given set repayment schedules.