From my interactions with industry and a couple of consultants, here is the list of reasons I could list out why consultants are hired:
1. Because it is fashionable to do so: Till about a decade or 2 ago, forming a committee was the managements way of assuring the rest of the world that the problem at hand is being dealt seriously by professionals. Today this role of committees and action groups is being replaced by Consultants and scores of PSU and govt bodies are hiring them.
2. There are a lot of tasks which are important, but are do mundane and distasteful that management is unable to find anyone in the organization willing to undertake it.
3. Reassurance and rally support: The promoter or the CEO wants a certain course of action. However he wants to look professional. Hence the consultants are hired and they need to find 101 reasons why that brainwave is good.
4. Cover Your Ass (CYA): It is easier to blame an external body for any ill advice in case the project fails rather than shoulder the blame yourself. Hence consultants double up as the fall guy
5. Resolve a Dilemma: Imagine a scenario: CEO with is 20-25 years of experience is clueless, the Board of directors are perplexed and the functional heads of the departments are unable to make up their mind about how to resolve a problem at hand. Here comes a bunch of blue eyed generalists who have no idea of what you do or what you want, yet they solve your problem.