Can you explain the difference between worldly and spiritual Guru service as described in the preface?

Chapter 0 distinguishes between worldly and spiritual forms of Guru service. Worldly service, such as writing biographies of great politicians, warriors, or writers, is acknowledged but considered limited. Its importance and fruit are not eternal, being constrained by space and time and often rooted in self-interest, thus yielding no spiritual fruits. Spiritual Guru service, such as writing the biographies of many saints, is a step above and does yield spiritual benefits, but is still considered partial. The ultimate form of spiritual service, as detailed in Chapter 0, is writing the biography of one's Sadguru with pure devotion, which is seen as the most certain path to the ultimate spiritual goal.