It's often more to provide backup and justification for a decision. Even if it then goes wrong you can say "Deloitte told me to do it".

While this may be true, it still doesn't take away two strengths consultants can bring: - never underestimate the value an independent view can bring, especially to a big company suffering from group think - secondly, a common consulting practice is to re-sell successful projects to other clients. Obviously not the exact IP, but the experience and general approach. This can be invaluable.