Earlier quoted context omitted.
One of the biggest challenges for large orgs is that the knowledge they need to do a given thing is almost always present inside the organization — but it's fragmented and scattered across different teams, roles, and balkanized administrative boundaries such that the big picture is almost never available. A high-quality consulting partner works to build the relationships and internal lines of communication that are n…
Critically, if someone within the org volunteers to organize and manage the cross departmental knowledge they'll be shut down by people within the company for trying to accumulate power or stepping into someone else's turf. Only an outsider can safely talk to everyone. They aren't a threat because they aren't part of the company and therefore can't take over.
Also, the end result and the path to get there isn't clear for FTEs – let's say they did the functional domain requirement gathering work – then who will implement it – they will have to contract out the development work – they may not be empowered to think and see through to the end goal fully.
On the other hand, for the top management folks – it is easier to temporarily empower the external consultants and terminate the engagement with them at any time – but they cannot do the same with FTEs due to employment laws etc.