Fungibility is unreasonably useful. Two things are fungible if they can be substituted for each other. The best example is of course money. A one dollar bill, a dollar coin, four quarters, or a credit card with a $1 balance are all essentially fungible.
Credit card payments don’t have to be made immediately so their value may not always be fungible with cash if the amount is large. The dollar bill and four quarters are more akin to each other than the credit card. If you have ever driven a car in an aging American city where parking meters only accept quarters or have slow credit card machines with dial-up modems, you understand the unique value of the quarter.
It’s obvious that fungibility always exists under specific conditions and fungible things can suddenly become diverse. Consider our fungible dollar assets. If you find yourself in a third wave coffee shop in San Francisco where they still don’t accept anything but cash, then your credit card, and dollar bill are no longer fungible.