What is the distinction made between worldly service and spiritual Guru service?

The Preface (Chapter 0) clearly distinguishes between two types of service. Worldly service, such as writing biographies of politicians or other great men, is considered temporary and limited by space and time. Because it is often rooted in self-interest, it does not yield eternal spiritual fruits. In contrast, spiritual Guru service, specifically writing the biography of one's own Sadguru and studying it with devotion, is considered the best and most certain path. This higher form of service is not partial and is capable of granting the ultimate spiritual goal of salvation.