A WBS is to a project manager what a recipe is to a chef.Just like a good recipe it sets out the required ingredients, equipment, methods, timing and performance indicators. Usually, they have a picture of what the dish is supposed to look like and indicate what it will taste like.Very important to making sure you are making the right dish.

Sadly, many projects get underway without a clear:
picture of what the project is to achieve
project methodology, and
understanding of the resources and their availability required to successfully complete the project.

Starting a project without a very clear WBS is inviting failure.