"A consultant will borrow your watch and charge you to tell you the time", or so goes the joke.
I've been a consultant with one of the largest professional services firms in the world. I think the single most valuable thing they can bring is an outside perspective.
Sometimes a company is so deep in their problems that they cannot see the wood for the trees. The people within the organisation may have the answers to the problems, but the company is structured in such a way that they cannot or will not listen to input from their own people.
An effective external consultant can come in and bring a fresh point of view to the executive level decision making process and effect change right the way through the organisation. Sometimes this is with new ideas and new ways of thinking, or sometimes it is with the same thoughts and ideas that people within the company have held themselves but for one reason or another have been unable to bring to bear on the organisation as a whole.
As mentioned elsewhere, there are different levels/types of 'Consultant':
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