There are generally three types of consulting firms.
They provide cutting-edge expertise in highly complex, unique, and possibly novel problems. They are hired by clients because they have the smartest people who can tackle any challenge thrown their way, regardless of whether they’ve seen it before.
Brains Consultancies often map (although they don’t have to) to the “expert” client-consultant relationship model.
They have experience solving problems in a familiar domain. Clients hire them because they are looking for reliability. The output is still tailored to the client’s needs, but it is less novel and cutting-edge than the work produced by a Brains Consultancy.
Grey Hair consultancies often map (although they don’t have to) to the “collaborative” client-consultant relationship model.
They have standardized processes for tackling routine, often trivial problems—as a result, these procedures can typically be followed by junior staff. This is often the case with accounting firms. Clients hire them when they want to outsource “boring” work.
Procedure Consultancies often map (although they don’t have to) to the “pair-of-hands” client-consultant relationship model.