It is important to get an outside opinion of your cyber strategy and defenses. While your organizational staff will likely have worked hard and spent long hours building robust cyber defense capabilities, an outside assessment will provide a good cross check.
Cyber defense is a rapidly evolving field, and it is difficult to stay current and vigilant even for teams with unlimited budget and staff. An outside verification process is like an accounting audit, providing a good feedback mechanism to the C-Suite and the Board of Directors that all possible avenues are being covered and cross checked. Internal organizations that get defensive about an outside verification process should be a concern, and prompt discussions about why an assessment makes people nervous. This is too important an issue to go on faith alone. Trust-but-verify is a good strategy.