Correction to the above, but there is also no way to make purchases online outside of the apple store, in that you don't have a number, a card, or any equivalent system of recording or verifying the existence of a card.
You'll note that this means that you need to purchase an iPhone even if you would not like a physical copy.
You're correct that you can use it on the apple store, but that's the only place you can use it. Apple pitched me on a general purpose credit card, at least equivalent to offerings from Visa or Amex. Only working on a single store is not in line with that, and in this day, not really in line with any standard expectation for such a card.
I expect that Apple understands that a credit card that can't make payments isn't functional, which is why I'm considering this a bug. They generally have a fairly high bar for products, and this is a low enough bar to meet that they cannot have failed to fix it unless they were unaware that it existed.