What important Mormon doctrines are taught in the book of Mormon that we couldn't find in the Bible already?

I've never seen a Mormon establish doctrine on the Book of Mormon, only arguing that the Book of Mormon is "another testament of Jesus Christ." So, since Mormons believe that is the case, surely they base some doctrine on the book of Mormon. So what important doctrines do Mormons believe are taught in the book of Mormon that we couldn't find in the Bible already?