I just don't understand this.

A national currency is an irredeemable medium of exchange, made valuable because it's -- by law -- exempt from capital gains tax (it measures capital gain), and because it's been given legal tender status.

Why would anyone trade an irredeemable currency issued by a private corporation? "Currency" is surely a misnomer, because no one would want to buy or sell goods and services in exchange for it, which makes it more like irredeemable equity, which makes no sense either.