Whale Rider creates a distorted medium between reality and the character's minds, which is what keeps the reader guessing, second-guessing and questioning all the way up until the moment of truth in the end. Ihimaera makes it the reader's job to presume Kahu's thoughts as opposed to actually providing them. Rawiri comes into play in that that he has had a special connection with Kahu since her birth, and being in a first-person perspective with Rawiri makes Rawiri like the reader in that they mutually want all of the answers.
If the novel were from the point of view of Kahu, the main focus, the plot would lose all curiosity. It was thought provoking and interesting to see Kahu strive for her grandfather's acceptance from before she could even walk. If Kahu narrated then we wouldn't be able to experience her development from birth, and would lose all of her toddler years. Those years keep Kahu a "question" and make us, like Rawiri, unable to wait for her to come of age and either become the whale rider or not.
Whale Rider carries itself primarily on motivation towards "the end". For the readers it is for the book's climax. For Rawiri, Nanny Flowers, and even Kahu herself, it is