Must be a reason but I'm to lazy to google "reason for the whole Jaredite thing."

I think the whole book was filler to pad the book. :)

Why?
Where else can you tuck in a story about everyone being wiped out?

Or That you can SEE GOD face to face like that!! How inspiring!

Or touch rocks to make them glow, because they didn't dare have a light in an air tight boat full of cow farts!!!!

And how else would the Malekites (who'd escaped Jerusalem with one of the crown princes fleeing from the Babylon invasion) meet up with the last survivor of the Jaredites as dire warning to the Nephites, Malekites, and Lamanites?

LOL It really is a bad novel, isn't it? Funny how we don't notice that while Mormon.

In Smith's day, there were two popular theories about where the Indians came from. One was that they were Israelites (usually, the Lost Ten Tribes). The other was that they were refugees from the Tower of Babel.

Smith wanted to cover both bases.

It did help him fill the space, too.

Smith or whoever wasn't a very imaginative writer when it came looking at society, technology, sociology or politics so he threw in 3 migrations from the old world to America and then also one additional colonization attempt (Limhi) story.

belief in the BoM will be when authorities announce that the Jaredite story was just a myth of the Nephites.

I like the idea above of covering both bases above, which I never heard of before. But I thought is was because in Mosiah some of the Nephites left to find their missing brethren in the wilderness and came across the bones of another race along with their records. Later J.S. thinks--well if this is the promised land and God leads only righteous people here, I now have to explain how this previous civilization got here by God's will.

Or, the Book of Jared was dictated by J.S. before he dictated Mosiah and he simply moved Jared towards the end of the BoM when edited and before the first publication.