Engaging a consultant usually means you have a problem that is deemed to require outside expertise.
This begs the question, “why would you engage someone who knows less about your business and its problems than you, to assist solving a problem?
The answer is simple, because the consultant knows less than you about the specifics, he/she is not constrained by the automatic assumptions that frame the way someone internal considers a problem, and is therefore able to ask questions free of the constraints of process, practice, and culture that develop in any organization. They are able to distinguish the wood from the trees because they do not wear the organization blinkers that internal people automatically assume.
Notice the emphasis on asking questions rather than giving answers?
A consultant who comes in and gives you answers should be shown the door, as he has just jumped to a conclusion on flimsy data. However, one who comes in and asks the difficult questions, ones that require a profound rethiknk of the status quo, is to be treasured. Such questions come from a breadth of experience in similar situations, and can lead to a solution that suits your organization by assisting you to come to the conclusions yourself through a process of helping you identify the impediments to the required outcome.
No consultant can know as much abnout your business as you, they just see it through different eyes, and from a less contrained perspective.