Since you are reading this book, it is most likely you. However, not everyone on a CMS project team will care about PAPI. Most of the predefined security roles, such as administrators, channel managers, resource managers, moderators, editors, authors, and subscribers, won't even be aware that PAPI exists. However, template designers, Web site engineers, application architects, and perhaps even information architects and operations personnel will be the key consumers of PAPI. If you have VS.NET on your desktop and are planning on coding using CMS, you will want to understand this API.
Unlike many of Microsoft's products, CMS does not have many wizards or shortcuts. Although it is true that CMS integration into VS.NET makes an excellent template-building environment, for advanced CMS work, you will have to rely on PAPI. For example, PAPI will be required to construct site navigation, summary templates, custom placeholders, and much more.