I just wish the financial industry itself had known about these when the core banking systems and financial communication protocols of the 60s and 70s were invented that are still being used to this day...

Many of these predate the widespread knowledge of idempotency, so often idempotency keys are hacked together by joining various, hopefully globally unique fields, except that they never quite are. (You can look behind the curtain sometimes, e.g. when your bank does not let you transfer the same amount to the same recipient account on the same calendar day.)

Also audit trails. Good audit trail can save company (and you) in emergency as well. Useful for debugging and last resort of compliance data source.

100%. It deserves more detail, too.

I've spent many hours explaining how idempotency is supposed to work, and why it's important. Most teams understand the need for it, but very few thought about it up front.