Can someone explain to me how and why consulting works? If a man had no real skills except giving advice left and right, he would be considered a loser. Now make it a company, and corporations and governments queue up for their advice. Wouldn't your own employees be in a better position to give you advice than people who know nothing of your business and who's only skills are googling and making presentations?
The ideal goal of consultancies like Deloitte's is that they hire a small number of people with experience and combine them with a larger number of young bright people and have them come in to review and advise organizations. The people with experience (so who have worked in that field before) direct the engagement and the leg work is done by the juniors, producing a report for the customer.
As to why companies would choose to use consultancies, there's a variety of reasons, some good, some less than good.
- Outside perspective & experience. The consultancy has done engagements with other companies in your field and can provide that experience.
- Neutral point of view. The consultancy should be neutral to any internal politics within the organization.
- Appeal to authority. Many times organizations use consultancies to provide evidence to external stakeholders that the thing they want to do is the right thing.
Now that's not to say that it always (or even often) works out that way, but in theory at least, there are some not terrible reasons.