He has not elaborated on his encounter with the Father before, but now he is trying to bring together the greatest and final harmony of his teachings. He is adding new information as an exclamation point to his commentary. Nephi would like his teachings to have our utmost respect. He certifies he is telling the truth. He makes a simple point: "Wherefore, do the things which I have told you I have seen that your Lord and your Redeemer should do; for, for this cause have they been shown unto me, that ye might know the gate by which ye should enter." (2 Nephi 31: 17.) He saw things, and he's telling us about them because they are true, and they are the only way for us to join him and Christ in eternal salvation.