The gift card analogy breaks down a bit: a gift card doesn’t get more expensive for the store to redeem later. Compute can.

Back when I experimented with getting ChatGPT by paying for API access rather than the official subscription, the credit was denominated in money, not in tokens.

Compute does whatever it does, but $5 sitting in an account to be spent on tokens at some arbitrary time in the future, earns money for OpenAI etc. even when they didn't make it expire.