It is usually hard to release resources for managing larger project from the own organization. Furthermore, the projects may become politicized as they proceed, which makes the execution even more difficult. An external management resource, with previous experience from a variety of executed projects, gives the initiative important objectivity and ability to carry out the project in a structured way.
Project management is all about managing actors and stakeholders. When this is done well, the project will advance as expected and the biggest surprises are mitigated. Running a successful project, does not even require a substantial expertise in the project itself, but rather the ability to grasp the big picture and to execute.
We´ll gladly tell more about our experieces from PMO- and project management.