Nephi already had the power to baptize. He had been ordaining others to this power and many of the Nephites were baptized under his authority (3 Nephi 7:24-25). Later, we learn that the 12 disciples and the multitude were baptized again (3 Nephi 19:12-13; 27:1). So why did the Lord give Nephi this power when he already had it? And why did the people need to be baptized when they had already been baptized?
The visitation of Christ represented a new dispensation, with a new covenant, a new law, new priesthood keys, and new ordinances. The new and everlasting covenant was being presented anew to a new dispensation. To borrow the language of the scriptures, old things are passed away; behold all things are become new (2 Cor 5:17). The new dispensation could not be founded on the old, albeit legitimate, ordinances of the Law of Moses. This is why the Lord said to the Pharisees, No man putteth a piece of new cloth unto an old garment, for that which is put in to fill it up taketh from the garment, and the rent is made worse. Neither do men put new wine into old bottles: else the bottles break, and the wine runneth out, and the bottles perish: but they put new wine into new bottles, and both are preserved…I receive not you with your baptism, because it profiteth you nothing. For when that which is new is come, the old is ready to be put away. (JST-Matt 9:16-21)