The book of First Nephi is a well-crafted piece of writing. Nephi carefully selected the stories he told, as well as which ones he left out. Thus, this discussion of his father’s dream is included for a reason greater than the simple question of whether it happened. Many things happened that Nephi didn’t report. Why include this information?
There are two reasons. The first is that this gives Nephi an opportunity to show how he was a teacher over his brethren. That is the fulfillment of prophecy, and a subtheme of the whole book of First Nephi.
The second is that Nephi’s audience was a people who had not grown up in the Old World and did not understand the stories and ways of telling stories from the Old World. It would have been a very unusual inhabitant of Judah in the days of Lehi who would not immediately recognize the tree as a representative of the tree of life. It is probable that Laman and Lemuel also knew it. It is unlikely, however, that Nephi’s readers would have had the immediate recognition that the Old World immigrants would have had. Thus, Nephi needs to clarify for his readers the images that might have been readily interpreted by Lehi’s family.