The ability to buy something does not equate to value of a currency. This is a pretty bad example but if I write a check to you for $10 I have given you a $10

The ability to buy something does not equate to value of a currency. This is a pretty bad example but if I write a check to you for $10 I have given you a $10 note. It's backed by my promise to you that I have the money. You cannot buy anything directly with the check. It's made out to your name. All you can do with that currency is trade it into a bank for another currency we all use. Another example may be gold. If I have you a gold bullion I hardly think you'd have an easy time spending it at a store (unless you're in Dubai). This doesn't mean gold doesn't have inherent value and it also doesn't mean gold is not a currency.