IBAN imposes a flexible but regular format sufficient for account identification and contains validation information to avoid errors of transcription. Routing errors were therefore frequent causing payments to be delayed and incurred extra costs to the sending and receiving banks and often to intermediate routing banks also.
A sort code is a number which represents a bank branch and intended for internal processing. Using data which is out of date can cause inconvenience to customers - their payment might be delayed or not processed - and it places a cost burden on businesses, individuals and banks as they rectify the problems which can arise.