The Book of Mormon clarifies doctrines that are not always clear in the Bible. For example, the Bible is not entirely clear on when a person should be baptized. Some have read the Bible and concluded that infants should be baptized or they will go to hell if they die. Others have concluded that this is not true. The Book of Mormon is very clear on this (http://scriptures.lds.org/en/moro/8/4-16#3). Until someone is mature enough to understand right from wrong, they are held innocent before God and do not need baptism. This is just one example of a place where the Book of Mormon clarifies doctrine. There are many others. However, there is another reason for the Book of Mormon; it serves as another witness of the divinity of Christ and the importance of His mission. It provides a witness from a completely different part of the world and shows that God is mindful of ALL of His children, not just those in Palestine. It shows that God is that same for everyone and that he will call prophets to teach all who are willing to listen to His gospel. If the Bible is the only source of scripture, then there were huge numbers of people who are condemned to ignorance of God simply because they did not live someplace where they had access to the Biblical prophets. The Book of Mormon shows that this is not the case. Rather all who truly seek will find God. Finally the Book of Mormon also shows us that the heavens are not closed, as some people would have you believe. It shows that God does still call prophets when there are people willing to listen to them. The printed word is great, but it can be twisted to say whatever the reader wants it to say. This is why there are so many different religions that are all based on the same book of scripture (the Bible). Continuing revelation from God is necessary to keep false doctrine from creeping into the body of the faithful. With prophets, God has a conduit through which He can say, "That's not what I meant," when people start to misinterpret what is is written in the scriptures. So, here are three reasons why the Book of Mormon was created. It is to clarify confusion that comes from relying on the Bible alone, provide a second witness of Christ, and provide evidence that God does still talk to those that will listen to Him.

The Book of Mormon came to be because after Jesus taught the people the same teachings to the new world as he taught those in the old world he commanded that the apostles in the new world write it down. He was also able to teach the new world inhabitants more because they were not as wicked. They didn't try to kill him. If your asking why do we need the book of mormon if jesus taught the same thing as the he did in the bible then you must ask why do we need the gospels of john, luke and mark, didn't matthew write all we need. If you don't know the four gospels all talk about jesus's ministry often repeating the same teachings of jesus. The more witnesses we have that Jesus was indeed our Saviour the more grateful we should be.

The teachings of the Book of Mormon are radically different from those of historic Christianity. It's not just a clarification, it's a difference. Now, Mormons are good people and good Americans and I hope Romney gets the vice presidential nomination. In fact he'd be a great President and I supported him over McCain. But you can't paper over the contradictions between Mormonism and Christianity. A comparison between them reveals many differences, such as the Mormon saying "as man now is, God once was; as God now is, man may become." That's absolutely anti-Christian. Or the idea that the Holy Ghost and the Holy Spirit are different. Or that Jesus was married, probably to Mary and Martha. Or that the dead can be baptized by proxy. Or that there are many gods besides the one God. Or that God has a physical body and that Jesus was physically begotten by Him. A Mormon will say that these differences are clarifications. Any well-informed Christian would say "Begging your pardon, but those aren't clarifications, they're contradictions." Dennis Prager, who is Jewish, explained the frequent hostility of traditional Christians for Mormons by saying that when a group that is close to yours in appearance but significantly different in beliefs claims that they are the same as you, or worse yet that they are the true believers and you aren't, it stirs up more hostility than to people who are completely different. So most Jews don't like Jews for Jesus, and most Christians don't like the idea that people who worship what seems to us a different God and another Jesus often claim to be the real Christians and the true believers. We think that Mormons, like Jehovah's Witnesses, are counterfeit Christians, and the more similar they appear (like counterfeit money) the worse it actually is. And, to answer your question, Christians believe that the book of Mormon was largely plagiarized from a novel about American Indians and that its other founding documents are pious frauds. There are good webcast lectures on this by "the Bible Answer Man".