Also, there are always choices to be made -- which is the essence of strategy. When faced with multiple choices, each of which appears fairly good, which direction should you go? A good consultant can help you spot hidden risks in some directions, or strategic long-term benefits in others . Similarly, when a set of choices appear equally bad, a smart consultant can help you choose the path that positions you best for a quick turnaround -- or sometimes can even identify a neutral or positive path that you hadn't considered.
As Peter Drucker put it, "My greatest strength as a consultant is to be ignorant and ask a few questions". Coming to any question fresh and unaware of real or imagined constraints makes it easier to see what people inside the company are likely to have missed.
To complement and expand on what has already been said above, when you run into a challenge that you have not faced before, you have several options, the main ones being - figure it out yourself, or bring in someone who has already figured it out - to help you address the challenge quickly and successfully. This can save you time and money, and help increase the odds in favor of your success.