Rhetoric: If we understand this text as a recorded oral discourse, then we should analyze it for its rhetorical structures, not its written qualities. In a discourse, the speaker must engage the audience quickly, as opposed to a written text that may be pondered more slowly. Nephi sets up a problem for his audience that has no simple solution. He expects that they will ponder the issue for a time before he presents the solution. In verse 5 he noted that the subject would be baptism, and that since the Messiah was to be baptized, so too they should be baptized. In verse 6 he sets up the problem. Why did the Messiah need to be baptized (using the past tense to reflect the surety of the prophecy that he would be baptized)?
In our modern understanding of baptism as cleansing from sin, we wonder why the sinless Christ had to be baptized. This would not have been the Nephite reference, as they lived nearly six hundred years before Christ and their understanding of Jesus was much more as Messiah than mortal. Their question would have been why the Messiah/Yahweh who is the author of atonement would have had to participate in a mortal rite of atonement.