This charter describes the mission, independence and objectivity, scope and responsibilities, authority, accountability and standards of the Internal Audit function.

A charter is basically a delegation or grant of authority from a person or organization of authority to another person or organization. The purpose of an internal audit charter is to compel the departments that need to be audited to provide the information required by the auditor. Without this charter or similar authority most managers wouldn't see any benefit to being audited and would likely refuse to provide anything the auditor needed.