Surprised you need to ask the question. The answer should always be yes and if the answer is no then the firm is not a management consultancy.
My own personal definition of a consultant is a party who adds value and knowledge to a company in a professional and engaging way. Thus anyone who doesn't doesn't class as a consultant,
In terms of benefit, I also have never undertaken a project where there has never been measurable benefit. How can you run a project when you have no deliverable? The deliverable is what in the final analysis performance is measured against, and the quality of journey is something that makes people return to you.
As an example we recently did some work for a large manufacturer/distributor (very well known). Turned them around, saved them circa £9 million for one operation alone and then worked with our client to win a major project with one of their previously dissatisfied customers. So pleased were both parties with our performance that our involvement in the client/customer project became a contractual obligation for both parties.