Is that a new TR question? Is it required to be LDS or even LDS in good standing?
It is a core, central truth claim of the Church. The entire Church is based upon it. If the Book of Mormon is not historical, than Joseph Smith was an utter fraud in claiming that it was, and the entire origin story of the Church, including Moroni, the Gold Plates, the Whitenesses, the translation, everything, collapses.
If the characters in the Book of Mormon did not actually exist, then they never experienced the things that book says they experienced. In that case, the book is purely fictional-and purely theoretical; it ceases to be a guide for the present because the crux of the Gospel-its application to actual mortal struggles with evil and opposition, becomes purely fictionalized. That is, made up. Religion then becomes a strictly abstract exercise in which lofty principles exist as little black marks on white paper but nothing more; the exploits and struggles of real people living real gospel principles against a background of spiritual insight, miraculous intervention, and the hand of God moving amongst his faithful servants protecting, guiding, and preserving them, is lost.
But that is, indeed, how so many want the Book of Mormon no? Many of us don't want those characters to be real; we don't want the Book of Mormon to be a record of real people living out a great drama against the backdrop of the continuance of the War in Heaven upon the earth and the Plan of Salvation because that historicity just brings the book and its teachings just a little to closes to home. That makes it too real,
too imminent, too down to earth. And we don't want our religion too down to earth do we, because that implies a direct confrontation with our own lives as parallel to those lives, historically lived, in the Book of Mormon.
Liberal Protestants have rejected Jesus as divine, the concept of miracles, and have come to cling to the "historical Jesus" for the same reasons: he's much more docile and much more purely symbolic. We don't want too much literalism in our religion because that brings it right to our doorstep.
The face of sin today often wears the mask of tolerance.
- Thomas S. Monson