Good question, even if you haven't heard the joke about the Consultant borrowing your watch so they can tell you the time. I promise not to answer using double-speak, jargon, acronyms, or fluff instead of substance.
So here goes: a competent Business Management Consultant is someone who is able to quickly assess opportunities to increase growth and/or reduce costs in an organization. It is that simple. Specifically, the opportunities might include identifying if the right individuals are in the right jobs; if products/services are properly positioned in the marketplace; if there are inefficiencies in the business processes throughout the organization; how the company's costs compare to competitors.
"Found opportunities" run the gamut from product design, raw material sourcing, location of manufacturing, forecasting, organizational structure (and the people in it), inventory practices, quality issues, manufacturing (and vendor) capacity, distribution, inefficient ("but it has always been done this way") processes, order fulfillment, and duplication of efforts.
A Business Management Consultant often employs a multi-step approach that goes something like this: obtain from the company executive(s) the underlying reason(s) for wanting the assistance of the Consultant (Objective); interview key individuals throughout the organization, and if appropriate, vendors and customers, as well as observe business processes throughout organization (Findings - which by the way, are frequently different from the hiring executive's understanding, resulting in employing extreme diplomacy not to offend when presenting!); analyze Findings, financials, industry best practices and key benchmarks of comparables to assess how the client stacks up and estimate the return on investment of opportunities (Conclusions); and lastly, taking into account a number of factors, determine the best way to maximize the client's resources and prioritize to-dos (Recommendations and Next Steps).
Often-repeated complaints regarding Business Management Consultants are that the Recommendations are infeasible, not able to be implemented, are so convoluted and/or simplistic they are useless, or basically are just a sales job for the next project for the Consultant. In some cases, this perception might be true.
However, I always follow the "get in, get it done quickly and to the client's satisfaction, and get out" modus operandi. I figure I would rather have the client call me back down the road than outlive my welcome (and value-add). I remember wondering as a newly-graduated Systems Analyst at Xerox Corp why there were so many consultants doing what employees should be doing. That wonder has stayed with me. Companies have amazing employees that need the bar set a bit higher -- who better to implement and own the Recommendations than the company's own team members. Of course, sometimes it is easier for an outsider to get the ball started, or provide an assist, which I am happy to do, provided there is a knowledge transfer along the way.