Justification of usage of Blockchain by banks
Blockchains are best used when there are multiple entities that don't necessarily trust each other. So within one bank, a blockchain really isn't useful. However between bank transfers could use the blockchain. Right now, in order to send between banks, banks use centralized systems (ACH, SWIFT, etc) operated by a third party. With federated blockchains, they can remove most of that third party's involvement and thus save on fees and time. That's really the only good use case for banks using blockchains that I have heard of.