Verse 15 continues the possible discussion over the meaning of "virgin," which might have signaled "young woman" more than chastity. If Nephi is operating on purely visual information alone, certainly he could ascertain that he saw a young woman. If he was also cognizant of her virginity in the modern sense of the word, then extra information was communicated to him. In the exchange with the angel, however, the sexual state of the young woman is irrelevant.
The critical information is her position as the mother of the Son of God. The angel does not rush into that statement, because he wants Nephi to correctly comprehend the import of that fact. The angel in verse 16 asks Nephi if he knows the condescension of God. Nephi responds that he knows that God loves his children, but freely admits that he does not know all.
The spirit again begins revelation with a question, a point on which Nephi must work to place his mind in the correct frame for the revelation which will come. In this case, the condescension of God and the birth of the Savior are intimately linked.
What is the condescension of God? To condescend requires a difference in status. It may only be done by one in a higher status, and indicates the person condescending physically or symbolically move from the higher status to a lower one. Thus it often has a pejorative tone, as we assume that the "higher born" condescend to deal with the masses, and that they do so unwillingly, or with some distaste.
There is none of that latter connotation in the condescension of God. His condescension is in the extending of his mercy to us who are undeserving of it. That mercy is embodied in his Son, Jesus. Hence the angel precedes the introduction of the birth of Christ with a question designed to place that birth in its important context. What Nephi is going to see is the greatest expression of the condescension of God - the greatest expression of his love for us.