The Lord gives a sign so we will know when "these things shall be about to take place." What are "these things"? They are the great Millenial promises contained in the preceding chapter, including the gathering of Israel and the establishment of Zion. The sign he gives is twofold, a) the coming forth of the Book of Mormon (v. 2), and b) the latter-day descendants of Lehi would know of their heritage through the Book of Mormon (v. 7).
These are two great signs that the work of the Father has commenced, but the grand fulfillment of all of the Father's promises can only take place after the Second Coming. Elsewhere, the Savior taught us another of the key signs by which we can know the time of his coming, And again, this Gospel of the Kingdom shall be preached in all the world, for a witness unto all nations, and then shall the end come or the destruction of the wicked (JS-Matt 1:31, italics added).