Hi guys so the best way to explain the situation is as follows:
You have 20 dollars. You need 2 dollars to buy breakfast. The merchant doesn't have 18 dollars to give you. It's extremely difficult to find 5 1 USD bills and 3 5 USD bills.
This may sound ridiculous but it is a reality that some people live with in, for example, Kazakhstan: https://youtu.be/3Ip5MvDMgcQ?t=817 – this English dude ordered some fried chicken for delivery and explains the difficulty involved with getting change for the delivery driver.
And then there are, of course, those in-betweeners. People who don't have 20 USD to spare to open and maintain a bank account with a minimum balance of 20 bucks.