When two banks transfer money, there's no concrete thing being moved or transmitted. It pretty much just involves bank B telling bank A "You hereby have $100k of my cash". So A needs some kind of global ledger to keep track of how much cash B actually has, and double check whether it's already told C, D, E that they have all of it.

There's no reason in principle a blockchain is better at solving this problem than a trusted central bank. But if the existing central bank is moving slowly or has annoying policies, it's much easier to bootstrap a blockchain than bootstrap a new trusted authority.