You find a sock full of quarters at the bottom of your closet. You take it to the bank. They give you a receipt for the $73.25 that was in the sock. A second later, the power goes off. Once everything comes back on, the computers reset to just before your transaction. Everything was preserved...except for your deposit.
Luckily, you have that receipt. It's your proof of deposit. It provides the formal record that you did, in fact, put your $73.50 into the bank. If you had lost the receipt in the few seconds it took for the power outage to end (which, let's be honest, would have been on brand for you), you might have been out of luck. Without the proof of deposit, the teller might not have been able to get her boss to update the system manually with your transaction.
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