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On the introduction page of the Book of Mormon it states that the Book of Mormon "is a record of God's dealings with the ancient inhabitants of the Americas and contains, as does the Bible, the fulness of the everlasting gospel." So, we do not discount that the Bible contains the fulness of the Gospel. The Bible went through many translations before we had it in the English versions it is in now and because of this there were important pieces of doctrine that became unclear. Even today, different Christian denominations will each have different interpretations of the same verses in the Bible. Some verses in the Bible will even contradict each other not because the Bible is not true, but because some pieces through translation and other doings of men have caused some precious truths of the Bible to be lost over several hundreds of years. The Book of Mormon and other scriptures work as another witness and help clarify many of the important truths found in the Bible. There are examples of doctrine found in these additional scriptures that is not clear enough in the Bible alone. Here are a few examples from my own experience: -Jesus taught that "except a man be born water and of the Spirit, he cannot enter into the kingdom of God" (John 3:5), and also set an example for baptism in the Bible. Christian denominations including us will disagree on how we should be baptized or how soon someone should be baptized after we are born. In the Book of Mormon when Jesus visited people on the American continent he made the mode of baptism clear which is baptism by immersion (3 Nephi Chapter 11: 24 - 26), and in the Doctrine in Covenants it states we should reach an age of accountability before we are baptized (D&C 18:42) which is eight years old (D&C 68:27). One of the Book of Mormon prophets also teaches strongly that infant baptism is not a correct doctrine in Moroni chapter 8. -The Book of Mormon also teaches what happens to our Spirits after we die and how we we will be resurrected (Alma Chapter 40). -The Doctrine and Covenants also contains revelations that are more applicable to people in our times. Even though God is never changing, people and the times we live in are. The Doctrine and Covenants contains revelation on how we should treat our bodies and what types of food and substances we should and should not partake of. (D&C 89) Along with this, in the Bible Paul teaches that our bodies are temples and that we should respect them (1 Corinthians 3:16-18). Other doctrines and revelations include: -Priesthood Authority, Ordinances, and blessings (D&C) -Additional revelation on Tithing (Malachi 3:8-11, D&C 119:4) -Teaching those who haven't had the opportunity to accept the Gospel in the Spirit World (1 Peter 3:18 – 19, 1 Peter 4:6, D&C). -Faith and Works (James 2:17-18, 2 Nephi 25:23) There are many other things as well. These additional revelations are important and necessary for our salvation although we believe that all will have an opportunity to learn and accept the Gospel of Jesus Christ either in this life or the next. I know this answer is kind of long but I hope this gives some insight on your question.