What problem can occur when you separate the write and read databases without using event sourcing?

Without event sourcing's strict, sequenced event log, an event bus delivering messages between a write database and a read database has no built-in guarantee of ordering. A later event, such as a withdrawal, could in principle reach the read side before an earlier event, such as the deposit that preceded it, since ordinary message delivery doesn't enforce per-aggregate sequencing on its own. This can silently corrupt a read model — a running balance computed incrementally in the wrong order produces a wrong result, with no error surfaced anywhere. Event sourcing fixes this by assigning every event for a given aggregate a strict, increasing sequence number the moment it's stored, so a projection always processes that aggregate's events in the exact order they truly happened.