Why banks require minimum balance

Banks are in the process of making money (of course). They make this money by lending it to others and charging interest, but they need cash and accounts for this to happen. Requiring a minimum is just to make sure there is literally money in the bank. Some banks have it to where you could go below the minimum during the day but by night it must reach the minimum. Others have an average monthly balance, mainly for the bank to see how their holding up their account.