Why is segregation of duties stronger than adding another review after the transaction?

Segregation of duties prevents one person from initiating, approving, recording, and concealing the same transaction stream. That matters because a fraud or error stopped before completion is usually easier to detect and less costly to unwind than one found later.

An after-the-fact review can still be valuable, but it is usually detective rather than preventive. If the same employee can grant discounts, process returns, and influence inventory release, a later review may catch only a portion of the problem. A better design is to divide authorization, custody, and recording responsibilities and then add exception reporting to detect unusual patterns.