While it isn’t the primary function of the central bank, one of the most important reasons for having the central bank is that it functions as the lender of last resort. If you want a loan you go to a bank, but where does a bank go when it needs a loan. To the central bank of course, and in the case of the UK this is the Bank of England. Commercial banks have historically been subject to runs, or panic withdrawals that can bankrupt a bank in a matter of minutes. When this happens the central bank stands ready to support the entire banking system.