Helaman is going to tell the story of these two thousand young soldiers, but he wants to give Moroni enough background to realize just how remarkable their story is. Therefore, he gives Moroni some background that Moroni may or may not have known.
The very first thing that Helaman mentions is that these young men were descendants of Laman: they were Lamanites, or at least had been. Although they were inheritors of the traditions of their fathers, they had spent most of their lives as Nephites. Still, the background of being descendants of the enemy, but taking up arms against that enemy, was important.