Can studying a Guru's biography serve different purposes? What is considered the highest motivation for this practice?

Yes, the Preface (Chapter 0) describes two distinct motivations for engaging with a Guru's biography. The first is 'Kamya,' which is undertaken with a specific desire, such as for worldly prosperity or the removal of worries, calamities, and diseases. The second is 'Nishkam,' which is performed with a selfless mind for the sole purpose of attaining liberation from the cycle of birth and death. The text unequivocally states that Nishkam Guru service, being selfless, will always be superior, whether it involves writing a biography or any other form of service.