Every number has to be explainable to someone who doesn't believe it.

Movement of money is a commodity; proving that every single atom of it has gone exactly where it was supposed to go is the engineering challenge.

Partial payments have to behave nicely and not create double charges or appear as missing in either of the ledgers involved. The reliable way to do this is to design the system around idempotency and compensating transactions, whereas exception handling is a poor alternative.

Auditing is a matter of reconstructing every step taken with every financial instrument. This is why immutability and event sourcing are critical, whereas a database snapshot of the “current” state is insufficient.

Finally, regulations change, reporting requirements evolve, and that innocuous hardcoded constant now needs to be justified in quarterly briefing packs to the auditors.