Confirm
What IS important about The Holy Book of Mormon is that it confirms: the Holy Bible, the virgin birth, the death and resurrection of Jesus Christ.
Unity
It seeks to unify the body of Christ by providing what was purposely excised from The Holy Bible, viz. instructions for administering baptism and communion, including the method of their ministering, who can receive them, etc., so there would be one faith and one baptism.
Testify
It seeks to testify to the House of Israel (and the lost sheep of the House of Israel) that Jesus is the Christ, and to bring them into the fold so there will be one fold and one shepherd, not two or more.
True Points of Doctrine