If you steal Green's gold watch, and then swap the purloined watch to me in return for one of my inspiring commercial law seminars, then Green can recover the watch from me in a legal action. It doesn't matter that I believed that the watch was really owned by you and that I provided the service in good faith and with pure heart. It's Green's watch, and your theft of it doesn't change Green's right to have it back. But if you steal Green's cash and pay me with that, no one would think that Green can get the cash back from me if I've taken it in a bona fide trade. And therefore, you're more inclined to take cash than the watch in payment.
Cash is also convenient, especially for small amounts.
And we now have cash on the Internet. Digicash, which has patents for its electronic cash systems, has teamed with Mark Twain Bancshares in St. Louis to offer its customers the benefits of a cash-type system that is entirely based on the exchange of intangible electronic information. But now we can as well. At least if the other party to the trade has a Digicash account and is willing to trade in the coin of the electronic realm. You can then swap these digital coins with participating merchants, who redeem them with the bank.