Project management is about making things happen.
A project manager is often a client representative and has to determine and implement the exact needs of the client, based on knowledge of the firm they are representing. A project manager is the bridging gap between the production team and client. So they must have a fair knowledge of the industry they are in so that they are capable of understanding and discussing the problems with either party.
Good project management is what makes the real work a success. Bad or missing project management can taint and nullify the efforts of even the most talented people. It doesn’t matter how brilliant your work is if the project as a whole is twice as expensive as intended, or a year late. This is not to say that the real work isn’t important—it is still the core of any project. No project manager can make mediocre work into an awesome end result. But fantastic work can be overlooked if the project management required to deliver the whole isn’t there.