Quick, Patch That Plot Hole!
Once Nephi and his brothers return safely (with Zoram in tow), Lehi gets pretty excited about the plates they brought back. And the audience discovers why the plates were so important. Apparently they contained the first five books of Moses and genealogical records of the descendants of Joseph of Egypt.
Oh, and by the way, the reason Laban had the plates was because he's also a descendant of Joseph.
This feels like something a modern writer who was not very skilled at storytelling would throw in when he realized that the motivations for his characters didn't make sense. This whole story took place and it was never explained why Laban had the plates and why he didn't want to give them away. Which apparently became clear to Joseph Smith when he had Lehi going berserk over the awesomeness of the genealogy the plates contained. He couldn't go back and put it in a spot that made more sense to the story because he's supposedly translating this by divine inspiration on the fly to a scribe who would probably think it was weird for him to be like, "Wait...go back the the chapter before and change this verse real quick." So he threw it in at the end.
During Lehi's plate-gasm, he makes a few prophecies about the brass plates. He boldly foretells that they will "go forth unto all nations, kindreds, tongues and peoples who were of his seed."
That's because it's a good chunk of the Old Testament. The Bible is a pretty well-known book. Um, a lot of nations, kindreds tongues and peoples know about it. Especially in North America, which is where a lot of Lehi's "seed" supposedly wound up. This is not a particularly impressive prophecy considering that Joseph Smith translated and published the Book of Mormon well after Christianity had been brought to the new world.
Lehi also states that the brass plates shall never perish. If he meant that they would never perish in the literal sense, then why don't we know what happened to them? Nobody has them. Where did they go? If he meant that they would never perish figuratively, that's kind of another prophecy that was already fulfilled when the Book of Mormon was published. Because the Old Testament still existed.
Who can imagine the Bible being lost forever? By the simple fact that the first five books of Moses are included in the Holy Bible, Joseph was assured that the prophecy he'd written was a safe bet--not a very impressive prediction. His attempt to make Lehi seem prophetic and wise fell flat.