A PBOK is a personal book of knowledge. It is the collection of your knowledge, ideas, and experience.
Creating, maintaining, and cultivating your PBOK is becoming increasingly more important. As automation and AI take over more and more jobs it will be creativity that will become increasingly more important. Your ability to take different ideas and knowledge and put them together in new ways is what will be valuable.
Your brain is not designed to keep all this information available. Your brain is more geared towards filtering information out while you are going after a goal. This is where your PBOK comes in. Your PBOK becomes your repository for knowledge, ideas, and experiences that are the fodder for your creativity.
Your PBOK becomes your secondary memory. Not having to remember your knowledge frees up your brain to be creative.