Why did a great soul like Sai Baba need to take on a physical form in the first place?

Chapter 43 clarifies that the incarnation of great souls like Sai Baba has a specific and benevolent purpose. He appeared in a physical form in Shirdi with the 'sole desire for the welfare of devotees' and for their protection. His appearance was prompted by the 'past merits of devotees' who were filled with devotion. The text states that the very purpose for the rise of such Great Souls is the welfare of people. Avatars like Baba are personifications of Brahman who appear to help others. Once their designated task is complete, they do not die but simply merge back into their true, unmanifest form.