The plates of brass contained the books of Moses and a genealogy of Lehi's family. They were important. They were integral to preserving their future and discovering where they had come from.
I think we have all wondered at one time or another how our life would be different without an influence in our life. Growing up I often wondered what it would be like to not be mormon. I think Lehi and Nephi saw the difference in what their posterity would look like with and without the Brass plates. Lehi also prophesied about the power and influence of the brass plates and how they would be used all over the world. Without the scriptures/brass plates, the history of the Nephites woud have been reduced to an oral history and perpetuated with errors like a game of telephone.