We remember the account of the baptisms of the disciples (3 Nephi 19:11-13). But, at that time, there had not been sufficient time to baptize the entire multitude. The ushering in of a new dispensation and covenant now required another baptism, even for those who had been previously baptized. This massive work was one of the first tasks of the new disciples. Apparently, the Lord truly blessed them with many manifestations of the Gift of the Holy Ghost, for we are told, many of them saw and heard unspeakable things, which are not lawful to be written (v. 18).
Joseph Fielding Smith
“Baptism, as we understand it, is one of the cardinal principles of the gospel, commanded primarily for the remission of sins, and, secondarily, as the door by which we enter into the Church… When Christ appeared to the Nephites on this continent, he commanded them to be baptized, although they had been baptized previously for the remission of their sins. We read how Nephi beheld angels who came and ministered to him daily; how he baptized all who came to be baptized for the remission of sins; how he organized the Church; and how he even raised his brother from the dead, since he held the priesthood (see 3 Nephi 7:15-25). Then we read that the Savior commanded Nephi and the people to be baptized again, because he had organized anew the Church under the gospel. Before that it had been organized under the law.” (Doctrines of Salvation, vol. 2, pp. 332,336)