The purpose of planning a project is to make sure it is completed on time, within budget and that it meets the customer's requirements. If the planning stage of a project is skimped, there may be major areas which have not received enough attention and which will have an adverse effect on the success of the project, no matter how good as the truism states "Fail to plan and you plan to fail".
An important principle of planning and controlling projects is recognizing the need for review of the plan. As the project advances the plan may need to be updated regularly to meet the changes in circumstance.
Next: Planning Process