[1] http://utminers.utep.edu/omwilliamson/ENGL1311/fallacies.htm
Is it a legal thing? European regulation tends to be quite aggressive when people get laid off, so I imagine that an external assessment helps give some cover for that.
Firing is somewhat more expensive, but if you want to close a location it is just an X million more not anything really complicated. Bigger chance the employees will be organized and fight in court, but in reality if management wants you out you will be.
The consultants are for the people who stay. You fire some group A, then you fire your consultations then group remain feels "Safe" again, when the bad consultants are gone.
However there are provisions that allow a company to lay off whole percentages of its workforce if the company is planning to face financial difficulties or similar. This has to be justified somehow so better get external reports to cover your ass.
In france, its a classic move by big companies to have mbb formalize what they already know: a need to restructure which unfortunately involves firing people. Its easier to justify restructuring because mbb said so than because the company says so for some reason ^^