What is the most important reason for applying the segregation of duties?

Segregation of duties makes it clear who is responsible for what.

Segregation of duties ensures that, when a person is absent, it can be investigated whether he or she has been committing fraud.

Tasks and responsibilities must be separated in order to minimize the opportunities for business assets to be misused or changed, whether the change be unauthorized or unintentional.

Segregation of duties makes it easier for a person who is ready with his or her part of the work to take time off or to take over the work of another person.