Comment by hasbroslasher

The example of your son's school is fine, but imagine that your son lives in another country. Cash has other problems beside being slow. It's insecure - a person could steal your $20 bill from your mailbox or his, or from the Post Office. It's untraceable - if your son's school says "We never got your payment" but your son swears he gave them the cash, you're stuck in a situation where either your son or the school is ripping you off (I'm sure your son would never do that). And last, it takes both time and money to convert cash to another currency. Taking the cash to somewhere where a person can convert it to the preferred currency is bad enough, and then it costs you money on top of it.