It seems more likely that it’s a politically expedient way to get cover for a decisionThat's certainly one of the reasons you would bring in consultants.
I think critics of consultants are, in general, too quick to dismiss the monetary value of political lubrication. If it's going to get your management hierarchy to admit there's a problem at all, or if it's going to get your management hierarchy out of a deadlock, that's worth something. If your organization is able to actually focus on the problems despite the politics, you are probably not a a typical McKinsey/Bain/BCG/etc. customer.
With maybe one exception, none of the consultants I know ever fit the mold of a fresh-out-of-college generalist who thinks their frameworks and raw intellect will help them come up with better answers than a specialist could. They mostly all understood that their jobs were to escort boring, good enough ideas through the boardroom politics. And that was often something they were good at, and they were often acutely aware that some people in those companies already had the answers but the organization was too dysfunctional to recognize the fact.
That's the core of the problem anyway. But naturally they'll sell you a lot of other promises and products, and those are where I think management consultants are the wrong answer. Just hire real specialists.
The other problem I have with management consultants is that I feel they, both the companies and many of the individuals, are too amoral to be healthy for society, but that's not about their competence.