It’s really about bypassing the existing power structure of the company. Competence of the work itself is a secondary objective. Most in-house initiatives can be slow rolled by management.
The fresh faced consultant with 2-3 steps to access the CEO neutralizes that. It seems grifty but is really exploiting bugs in corporate governance.
The current fad of firing the managers is a riff on this. Every jackass C-level is coming up with the novel idea of flattening.
Most of the times, the business decision has already been made long before McK is hired. It’s all about legitimizing that decision and making it happen.
You can also wield them as a weapon against internal competitors or opponents. Look up how they were used to kill off Cariad for example.