Jesus officially institutes the ordinance of baptism among the Nephites and gives Nephi and others the authority to baptize the people. In Mormonism, baptism is considered an essential ordinance and must be performed by one who holds the sacred priesthood. Interestingly, one of the few bits of advice Christ gives on the issue of baptism is the clear warning that there should be “no disputations” about the issue, an admonition he repeats in verse 28
Mormons believe that the Book of Mormon is intended for our times and contains those specific teachings that would be most relevant for today, so Mormon, the editor, may have chosen to include this bit of Christ’s teaching (out of many others that went unrecorded) because contentions over baptism have characterized some modern Christian groups.