Based on a very brief conversation I had with a Paypal engineer a few years back, it's SOP in modern financial transaction handling to store/log details of ever

Based on a very brief conversation I had with a Paypal engineer a few years back, it's SOP in modern financial transaction handling to store/log details of every transaction in many places and various forms, and there's plenty of (probably excessive) paranoia involved. The context of the discussion was performance, scalability, and engineering resources, so we didn't get into how the logs are used, but it would be my guess that if the "primary" database had some sort of failure that lead to a rollback, affected accounts would be locked out until the transactions can be properly recovered from other storage/logging mechanisms.