There are many reasons why an author would use first person, but at first it may seem a bit strange that Butler would use first person when she used third person in the first two books of her trilogy. Upon a second look, however, it is apparent that the choice to use first person was entirely calculated and deliberate.
Throughout the first two books, it is very difficult to get into the head of one specific character. The reader is merely told, or shown through action, what the humans and Oankali think or feel. There are no first hand thoughts or actions.
Once the reader progresses about a quarter of the way through Dawn, he is even able to predict the behavior of the Oankali, especially specific ones to whom the reader has made a connection. He would be more easily able to predict Nikanj's behavior, for instance, than Kahguyaht. This ease of prediction for certain Oankali is usually a moot point for most Oankali act the same way and have the same priorities. They only become unpredictable, and only an almost negligible amount unpredictable when they are dealing with the humans to which they have bonded. For instance, after Joseph is killed, one can predict that Nikanj would try to comfort Lilith even though she "want[ed] desperately to be let alone" (225). Of this want to comfort the reader is positive, and of Nikanj's need to experience everything with Lilith. What takes the reader off guard is when Nikanj "gave her. . . a new color. A totally alien, unique, nameless thing, half seen, half felt or. . . tasted. A blaze of something frightening, yet overwhelmingly, compelling" (226). That fact that Nikanj has finally given Lilith something of itself, it's feelings, is the first no predictable thing it has done. That is to say, once the reader has understood the behavior of the Oankali, something like the sharing of emotions so intensely like this seems out of place.
All this is to say that the reader can reasonably predict actions of characters but does not know any direct thoughts or feelings thereof. This lack of personalization is the main reason Butler chose first person for this novel. Because Jodahs is a construct, it is an entirely new form of creature, a combination, neither fully Human or fully Oankali, and therefore unpredictable to the reader. It will be easier and more beneficial to the reader to be able to access its mind directly. At the beginning of the novel, the reader is introduced to Jodahs through his own thoughts and feelings.
"I slipped into my first metamorphosis so quietly that no one noticed. Metaphorphoses were not supposed to begin that way," begins Imago. This first section holds a lot of significance. It presents the novel as written in third person. It presents the narrator as unusual because of his unusual entrance into adulthood, foreshadowing for the rest of the novel. It also allows the reader to quickly identify with the narrator and form a connection or a bond with him. The beginning of the novel consists of a lot of Jodahs telling us about his thoughts and feelings. It shows the reader the thoughts of a construct. "But I did not feel drawn to him as I should have," Jodahs tells the reader. "Nor did I feel drawn to Lilith, my birth mother. She was Human, too, and what was happening to me was definitely not a Human thing" (524). This confessional type reading allows the reader to feel a deeper connection with Jodahs than they had felt with Lilith or any other characters in the first two novels. That passage also hints at a strange sort of aversion that Jodahs' position as construct has given him to his Human parents, as if he doesn't want to subject them to anything more alien. Or, that he doesn't think they can help or understand at all. As the novel progresses, and the reader gets to know and understand Jodahs, the reader gets less and less of that intimate, inner dialogue, allowing for us to speculate and predict its emotions a little bit.
Butler uses the first person to create an intimate connection with construct Jodahs. By seeing its inner thoughts and feelings, the reader gets a richer view of Jodahs and the world and people around it. It also allows for more of an emotional connection to it.
Butler, Octavia E. Lilith’s Brood. New York City: Grand Central Publishing, 1989.