Managing successful projects and programs depends on knowing what DONE looks like. One of the views of DONE is the Work Breakdown Structure (WBS). The WBS describes the products or services delivered by the project and the processes that produce those products or services. These process are NOT things like "design," "test," "code." They are the manufacturing, fabrication, testing and evaluation processes. Many outside of the MIK-HDBK-881A work confuse (sometimes intentionally, sometimes just to be different) the differences between a functional breakdown structure with the WBS. There is only ONE definition of the WBS that worth anything and that is the MIL-HDBK-881a description. All others, including the PMI advice is derived and should be considered a secondary source.
As well, there are domains, where the WBS - a good WBS - is seen as an option. This "optional" approach is coming to an end. Standardization of Work Breakdown Structures to Support Acquisition Management, will change the 881 Handbook to 881 Military Standard and make the development of a "good" WBS mandatory for all programs.
Many in IT project management business woudl be well served to learn how to build a WBS and use it to describe DONE. That way when DONE arrives it will be recognized.