During the lifecycle of a project it is vital that team members and stakeholders have a clear path of communications. If communication paths are not kept in check they will rapidly grow out of control. The communication plan spells out communication paths, appropriate mediums for communication and appropriate intervals for communications.
The project manager must make sure during the construction of the communication plan that all communication needs are taken into account. What will the stakeholders require on an ongoing basis? What information will they need to be comfortable about the progress of the project? How frequently will they need updates on status, risk management, forecast completion, etc.? Defining these types of communication will reduce the amount of ad hoc communication necessary once project execution begins.