“Wherefore” shows that Nephi is linking Jesus’s action in submitting to baptism to the descent of the Holy Ghost. The particular actions that appear to be linked are the reasons for his baptism. Just as with his baptism, Yahweh incarnate would have no need of the gift of the Spirit. However, as a model for mortals, it is an essential part of our acceptance of the atonement and our ability to be attuned to its continuing impact on our hearts.
While the association of the Holy Spirit and baptism may seem to be a Christian concept, baptisms at Qumran also apparently invoked the Holy Spirit. Robert Eisenman and Michael Wise, both of whom played key roles in opening up the Dead Sea Scrolls for publication, discuss a fragment of a text on baptism at Qumran and note:
On the heels of this text, we come upon a series of fragments relating to baptism. By baptism, of course, the reader should realize that the proponents of this literature did not necessarily mean anything different from traditional Jewish ritual immersion. The terminologies are synonymous, though the emphasis on baptismal procedures at Qumran is extraordinary. This can be seen not only in texts such as the one represented by these fragments and the well-known Community Rule, iii, 1-4, which in describing baptism makes reference to “the Holy Spirit,” but also the sheer number of ritual immersion facilities at the actual ruins of Qumran—if these can be safely associated with the movement responsible for this literature.
While the scrolls community developed about 200 B.C.–A.D. 70, four centuries after Lehi’s departure from Jerusalem, some texts precede by about two hundred years the Christian practice of baptism and conferral of the Holy Ghost. While this pre-Christian connection between baptism and the Holy Spirit cannot be firmly dated to the period when Nephi could have encountered it, baptism and confirmation should not be precluded prior to Christ’s birth—particularly since Nephi is declaring these things as part of his own vision.