* It can be accepted without the help of technology
* It can be stowed away in a secure place (in contrast to money on bank accounts, which can "evaporate" in case of a bank failure)
* It is very hard to trace and provides a good level of anonymity
* It can be sent through the mail or transported physically form one place to another
And on the less serious side:
* It can be piled up into a giant mountain and burned
* It can be rolled into nice wads and used as props in ganster movies (ever seen a gangster wave around his black Amex in a film?)