One of the biggest challenges for large orgs is that the knowledge they need to do a given thing is almost always present inside the organization — but it's fragmented and scattered across different teams, roles, and balkanized administrative boundaries such that the big picture is almost never available.

A high-quality consulting partner works to build the relationships and internal lines of communication that are necessary for the client organization to do more of the work — maybe all of it! — in the future. A cash-grab consultancy will just make themselves the organization's "glue" and bill until the money runs dry.