There are two primary reasons why separation of duties is important within every financial process. People are an integral part of every financial process. They authorize, generate, and approve each transaction. Having different people engaged in critical positions within the process is a common, sensible business practice that ensures consistent and successful execution of the process. People have their shortcomings.
People are human which means they occasionally make mistakes.
The complimentary check by another individual means that there is an opportunity for someone to catch an error before a transaction is fully executed and before a decision is made based on potentially erroneous data. Sadly, a few people may be inclined to engage in fraudulent activities. Justification a sense that they have not been treated fairly, the employer owes them, etc. Having adequate separation of duties reduces the opportunity factor that might encourage an employee to commit fraud or to embezzle. The person who reviews the transaction for accuracy, validity, appropriateness, and proper authorization.