The work itself was generally not all that good. The knowledge of “experts” brought into meetings rarely contributed more than what a reasonably intelligent person could dig up on Google search results in an hour. They were also often farmed out on random staff augmentation functions that just annoyed the hell out of people. “Hi I need you to fill out this excel spreadsheet with 35 columns so we can put a presentation together... oh and if you could do that by 6 PM tonight that would be great.” That sort of nonsense so they could produce some nonsensical 50 page PowerPoint deck that nobody read.
There were a few decent people there but by and large value was not generated. As others have pointed out a major motivator seemed to be to provide some C-level exec with CYA coverage to claim that programs being implemented were based on the advice of outside “experts.”
In the two main cases I saw the McKinsey strategy ended up being a total disaster that seriously damaged the company and the C-level exec that hired them in both cases got canned as a result so in the end even the CYA concept didn’t really work.