Because the WBS will give you a view about WHAT you need to deliver, while your planning will give you a view on WHEN and HOW you will do it.
One thing to keep in mind about building a WBS is that is an interactive and iterative process.
I go through this process with an example in my article “How to create a product oriented WBS”, what is a project management best practice.
In a nutshell:
- Identify the main products or deliverables to have your first draft
- Ask to your team or stakeholders WHAT needs to be done to deliver the main products or deliverables
- Continue with the analysis until you identify additional levels of subproducts
- When no more subproducts can be identified, your WBS is ready and you can start to prepare your planning
- Remember, the WBS is not something you have to build alone, the most you involve stakeholders and the team, the most common understanding on WHAT needs to be done you will get