Self Assessment is a form of checking the quality of your processes and functions. the power of self assessments lies in the following aspects:
Its done by people on the ground: who knows your processes better than the people who are dealing with it on daily basis? that's why self assessment equip these people with tools and techniques to understand, examine, propose solution and improve their work.
its a built in function: once you set up your self assessment, its will take place every time seamlessly without your intervention given that you self assessment is set to match your process time frame to ensure it runs when most information is available and there is time to implement the changes.
it boost morality: when your people assess their work, it gives them sense of ownership and pride in their work, improvement becomes a goal they aspire to achieve rather that imposed by senior management.