[ThrowAwayIRUK]

>> ..In most countries, payments within the same bank have been instant (and free).. <<

Which is quite simple: First, its internaly only an exchange of liabilites in your balanace sheet.

Second, it does not involve any outgoing gateways/interfaces.

Third, if you are using a SaaS-corebanking-solution, those types of transactions are usually excluded in your contract.

From that perspective, it is absolutely useful.