The primary reason to perform internal QA for an internal audit function is to ensure that the internal audit activity adheres to the Definition of Internal Auditing and the International Standards for the Professional Practice of Internal Auditing (Standards) issued by the Institute of Internal Auditors (IIA), as well as the internal audit methodology and policies of the organization. A QA program enables an evaluation of the internal audit activity's performance, efficiency, effectiveness, and value, and identifies opportunities for improvement. A QA program also helps to enhance the credibility and reputation of the internal audit function among the stakeholders.
References
Quality Assurance - The Institute of Internal Auditors or The IIA
Benefits of a quality assurance review for internal audit
Optimize your internal audit function with a quality assurance review ...