Why engage a consultant
The usual reason to hire a consultant is to get access to some of their skills that are scarce or nonexistent in your own company. Beware that the more narrowly a consultant defines himself or herself (e.g. "I develop mail-order catalog website back-ends with Ruby on Rails"), the less helpful they will be. When you hire a button-pusher, don’t expect miracles — expect buttons to be pushed.
This is the original and best reason to hire a consultant, and where ‘miracles’ can occur. Real consultants are people you can consult. Sadly, the art of asking good questions, gathering information about problems and giving advice based on experience and observations has almost become a lost art among today’s see of ‘consultants’, who technically should call themselves independent contractors, not consultants.
"The imaginative user of "outsiders" can encourage each of the types of mindfulness just mentioned. […] Just as a traveler to a foreign culture notices what people indigenous to that culture take for granted, an outsider in a company may notice when the corporate natives are following what may now be irrational traditions or destructive myths."
— Ellen J. Langer, Mindfulness