If you are wanting to improve and develop your competence and capabilities as a business for the future - fronting up to your blindspots and wayward assumptions is a critical part of that journey.
Putting it another way...if the people around you just tell you how brilliant you are all of the time, and never critically point out where you might improve, then you will never know where the gaps in your performance lie.
Praise is worthless, provocation is golden.
The problem with agencies (ad agencies, marketing agencies, management consultants etc. etc. etc.) is that their continued stream of revenue largely depends on keeping their clients happy.
They package their recommendations and services based on what they think their client wants, rather than what their client's business really needs.
An agency will typically avoid highlighting problems; they'll shy away from pinpointing a senior director as the primary source of the company's ongoing issues (even if they know it to be true), they package their recommendations as simply as possible (making them look good even if the dumbed down version doesn't account for the complexity of the issue) using lovely big pictures and a pleasing font that doesn't rock the boat.
This approach to working with a client is not really what's best for the client's business, but rather is what is best for the agency's business.
Moving forward requires provocation; the brutal truth, uncomfortable conversations, a spotlight on the real barriers that are holding progress up. In some cases the problem is the person paying the bills, meaning that resolution requires risky conversations.
Unless an external service provider is bold enough to, and encouraged to, play bad cop on behalf of the future of a business, then all they are, are pointless cheerleaders and a perfect echo-chamber for an existing culture of bullshit and denial.
Progress is always messy and slow and hard and difficult to stomach, but without honesty, any superficial short-term gains will just eventually lead everyone straight back to the starting-point yet again.
External agents are useful because they outside of the organisation, so should be immune to the politics of the organisation. When their blind agreement assures their own survival, their value is nullified.