Nephi introduced the complaining brothers in order to show how he taught them. This part of the story is not told simply because it happened, or because no other important events occurred. This event is told because it furthers the story of how Nephi was a teacher over his brethren.
Nephi set up his theme in the previous verses where Laman and Lemuel did not want to leave Jerusalem. One of the subthemes of 1 Nephi is how Nephi paints his family’s journey as a recreation of the Exodus account. Thus, he notes that Laman and Lemuel did not want to leave Jerusalem, and the lesson he teaches them is about the children of Israel who were in bondage in Egypt and needed to leave.
Laman and Lemuel certainly would have not thought the two situations entirely comparable, as they were not in bondage in Jerusalem as Israel was in Egypt. As they noted, they had possessions and could have been happy. That, of course, is not Nephi’s point. The point is that they were commanded to leave, as was Israel.
Although there were differences, by likening Lehi’s family to Israel’s exodus from Egypt, Nephi reminds the brothers that there is a promised land awaiting them, and that it will be better at the end than it would have been if they had stayed, and better than it has been during the journey—just as it had been for Israel.