The document will form the definitive description of the aims of the project and, as such, can be used to manage the expectations of the stakeholders. It will also include acceptance criteria that will ultimately be used to judge whether the project was a success.
The tasks need to be scheduled in the correct order and dependencies between tasks factored in. In complex projects several tasks will be performed in parallel to maximise the total project time. The plan will also take account of the project budget.