Project Management Body of Knowledge (PMBoK) is a collection of often-used practices for project management. A project manager doesn't have to use all of them. An organization is supposed to adapt PMBoK for its needs. So why do we have The Standard for Project Management included in PMBoK then?
3PMBoK says that the guide itself and the standard are "descriptive" rather than "prescriptive". Are you asking because you consider The Standard for Project Management to be a set of prescriptive step-by-step instructions on how to do project management... or?!?!– BogdanDec 1, 2020 at 20:02
I remember one person saying that "these things are like 'right of way' rules when you drive your car." They don't tell you how to drive your car, but they do tell others what to expect from you as you drive past them, and you what to expect from them as they drive past you. (And of course, by doing so, "they avoid a lot of collisions ...")
These things provide a consistent nomenclature, a consistent conceptual model to work from, and a consistent pattern for interaction between parties – even (and maybe, "especially") when said parties do not routinely interact. They are also a set of "best practices" that have stood the test of time in many different contexts.
The first benefit of PMBOK is that it allows businesses to standardize processes across departments. This means that project managers in development work in the same way as those in distribution. Second, PMBOK can assist project managers in using a consistent framework across organizations.
WHY? The standard accomplishes it's purpose by laying out detailed concepts and instructions for sponsoring, directing, and managing projects. It includes project planning and execution, as well as project management techniques, project management language, and business-related project risk.
A guide book is not the same as a standard. The publication called "A Guide to the Project Management Body of Knowledge" is just that: a guide to many aspects of project management published by PMI. The PMBOK Guide is not a prescription for how to do project management.
Various bodies create and approve standards and ANSI is one such national standards body. In 1999 ANSI approved the standard that is included in the PMBOK Guide. There are other standards however, notably the ISO 21500.