> Why?

A non-hypothetical scenario is that I make a credit card payment via ACH. That money does not leave my account until EOD. Some other transaction causes my balance to fall below the amount required for that transfer. The transfer fails and I am dinged for a missed payment by the CC company (and if the bank sucks they charge a fee too).

Now you could argue this is an accounting issue "Why doesn't the bank reserve the funds until the transfer is complete?" Answer, I don't know ask Capital One. But RTP eliminates the question entirely.

This is a thing already in the UK. I can make a credit card payment from my account using the credit card number. It applies to the balance in real time (realistically in minutes). I don't have to worry that the payment could fail at any point.

>Now you could argue this is an accounting issue "Why doesn't the bank reserve the funds until the transfer is complete?" Answer, I don't know ask Capital One.

Because transaction ordering isn't guaranteed. Say I know that I'm getting paid on the 1st, and I want to spend my paycheck immediately on rent. If the rent transfer gets processed at 11:00 and the payroll provider gets around to doing it at 11:15, I don't want my rent to be rejected.

Therefore, the bank allows transactions to happen and only checks at the end of day for overdraft. They then apply a time-independent ordering to transactions they can reject like paper checks.

In fact, this same scheme applies to banks, Fed only looks at their accounts at end of day, why you have overnight funding.

The major exception to this is debit cards, which Congress made them reject if the money isn't there at time of authorization.