The internal audit unit is independent of the units it audits so that it can act as a check on them. The Basel Committee on Banking Supervision's 1998 principles say that the internal audit function should report directly to the board of directors, the audit committee, and senior management.
In the three lines model, verifying compliance is in principle the job of the first and second lines within their own operations. Internal audit, the third line, takes the state of that verification into account and independently and objectively checks whether the controls are sound.