In the industries I've been in...it is usually top execs bringing consultants in...and it is for two primary reasons - 1) the idiots under them that suck at the financial performance responsibility aspect of their job descriptions can continue to suck at the financial performance responsibility aspect of their job descriptions because they are good cheerleaders for the executive vision and mission which is full of the latest and greatest progressive buzzwords, useless socialist-engineering team building events and sundry other wastes of productive time (thus sheltering this group of favorite pets from real responsibility or criticism or accountability) and 2) to lay the burden of implementation and performance on everybody else without having to admit either their culpability in creating the problems in the first place by their actions (their hires, their policies, their decisions) and avoiding the unpleasant task of holding their pets accountable when the consulting process and failure to achieve results can find the appropriate scapegoats. Bottom line it is chickenshyt leadership. And the more expansive it is the more likely it is that it will make everything worse. This isn't to say that it is universally true all the time, some well led well managed outfits with good personnel top to bottom can benefit from some fresh eyes and some sensible suggestions...but even they can screw things up by hiring the wrong outfit, focusing on a problem that isn't a problem and avoiding an area that is a problem, etc etc.
In general my experience with consultants is mixed...some a total mess at inception, others benefiting only in parts...the ones most likely to succeed are limited in scope, specific to need, understandable and achievable in goals/processes/methods and seen by directly affected operational staff as worthwhile attempting.