Professional project managers bring extensive experience and knowledge based on thousands of hours worked on successful projects. A good PM must have the ability to apply frameworks and methodologies, such as agile and SCRUM, throughout the project lifecycle.
If team is implementing a project in a new field or a it is a totally new case for them, adding a project manager with past experience will be critical to the success of the project. Otherwise, the project may be more time and cost consuming.
An experienced PM can prove to be a time saver, helping with timely approvals, defending change requests and areas needing immediate improvement. His expertise in understanding complexity and making quick decisions can sort unwanted show stoppers on the way to a successful project.