The Project Manager as the title suggests, managers your project. He ensures the builders and tradesmen attend site in the right sequence and all work is accurately built according to the construction plans and meets the National Building Code. A self-managed fit-out would require you to take on this role, and while this might save costs on paper, it wont in reality especially when time is considered.
Any changes or amendments to the design are managed by the Project Manager as well as requests for periodic payments. They will be responsible for the communication between you and all other parties involved in the project.