> Is there a reason why you wouldn’t pay ahead of time? I just understand why you couldn’t buy a few dozen/hundred/thousand dollars worth of credits, and wait until it runs low.

Maybe you want to buy the service that is priced the lowest at the moment. Example: providers of inference services could drop their prices if they are underutilized. You could then have your system check for the best price and purchase only what is needed at the best price.