The WBS lays out all the small tasks required to be done for the completion of the overall project. In order to have a plan set for all aspects working together, which is necessary for product development, the WBS is crucial. Like you said, the WBS is not a replacement for other documents necessary in planning phase, but it is a crucial part of planning. The WBS will allow project leads to set timelines for each task, and accurately predict progress and budgetary progress. Overall, the WBS is an essential part of the planning phase in successful product development.