An overall project plan allows you to collect, organize and sequence the high-level tasks that need to be accomplished for the deployment to be successful. It will also get you thinking about who will perform the various tasks, and what dependencies may arise between tasks or between team members.
You create the project plan early in the project, but you will continually refine your plan as you move through the six phases of the deployment. At the end of the project, it's a good idea to review the project plan once more, noting down areas that were missed or that needed more attention and up front planning.
