Don't leave just yet, I've got an answer to that question.
God knew that this was a pretty amazing story that would need some serious backing up. Gratefully, He didn't just leave us to ourselves to determine if it's true. God knew that after so many centuries of people being confused and in darkness, we would need some convincing evidence.
And in steps the Book of Mormon.
A few years after having this remarkable vision of God the Father and Jesus Christ, Joseph Smith began to receive more knowledge and direction from God, preparing Christ's church to again be restored to the earth. Part of this outpouring of knowledge was a visitation from a heavenly messenger named Moroni. Moroni directed Joseph to an ancient record, written on thin gold plates, that had laid hidden in the earth for centuries. After years of preparation, Joseph was able to obtain these plates and translate them by the power of God. As he did so, a wonderful story began to unfold.
Back in the time of the Biblical prophet Jeremiah, there was a man named Lehi who lived in Jerusalem with his family. Lehi was also a prophet, and he received revelation from God to lead his family away from Jerusalem (because, you know, it was about to be destroyed and taken captive into Babylon). Obediently, Lehi did just that. He and his sons and daughters traveled through the wilderness, eventually reached the ocean, and sailed across it to a promise land. On this land, which we now know as the American continents, Lehi and his family began to populate the land, and over the next hundreds of centuries (about 8), these people became great nations. They had times of happiness, famine, war, and peace. They communicated with God through their own prophets (remember, they couldn't just skype Jerusalem and see what was up with Malachi), and prophesied of a Messiah, Jesus Christ, who would come to save His people. Throughout it all, these people kept detailed journals. The crowning event of all these records is around the year 34 A.D. when Jesus Christ himself appeared to the people, ministering to and establishing His church among them. By the year 400 A.D., however, these people had rejected the gospel and fallen into a deep apostasy. One of the last righteous men to live among them, Mormon, complied all the records onto gold plates. He passed them to his son, Moroni, who then buried them in the ground, asking God to protect them and bring them forth in His own due time.
Fast forward 1400 years, Moroni appears to Joseph Smith and directs him to these records. Through the help of God, they are translated and published as The Book of Mormon. They provide a further testament that Jesus is the Christ, the Holy Messiah, and teach us God is aware of His people all over the world, and not just in Israel.
So why in the world is this important? Why was the Book of Mormon even needed?
Let me break it down for you.
The Book of Mormon helps us know if Joseph Smith really was a prophet. The Book of Mormon is a book of scripture that only this church uses, and it is something we claim was translated by the power of God. Without it, we are going on the word of Joseph Smith actually he actually had authority from God and was a prophet. With it, we have physical evidence we can hold in our hands. Yes, we have the Bible, but if you read the Bible and gain a testimony it's the word of God, then what? You know that Jesus is the Christ, so that might help you realize you need to join a Christian church, but which one? There are hundreds, if not thousands, of Christian churches out there, and only one of them has The Book of Mormon. By default, if the Book of Mormon is true then the Church of Jesus Christ of Latter-day Saints is also true. It's the domino effect.
The Book of Mormon helps to restore lost truths. Remember how we talked about all those changes the "house" (Christ's church) went through? How the blueprint started to get changed as it passed through the hands of countless kings and spiritual leaders? Gratefully, the Book of Mormon laid untouched for centuries, and has only been through one significant translation. Because of that, the truths it teaches and pure and undefiled by men. It teaches us more about God's Plan of Salvation, and how we can have happiness here on earth and prepare to live with God again. It's sort of like an elaboration on the Bible, those parts that sometimes get really confusing. I may read a verse in the Bible and sense there is some principle being taught, but it's only alluded to and not really explained in detail. As I then turn to a passage in the Book of Mormon that talks about the same principle, I suddenly gain a better understanding of what Paul or Matthew was trying to say. My faith in God grows, and I better understand the gospel.
The Book of Mormon provides another witness of Jesus Christ. Let's say you're judging a court case. The prosecuting attorney brings a witness to the stand, who shares their experience with the subject at hand. And then they bring in a second witness, who shares their experience, and these two witnesses happen to be very similar. This helps you to believe the story of the first. So tell me, if you had the one witness, are you going to turn down the second just because you've already got one? No. The same is true with the Book of Mormon. The Bible is split into two sections - the Old Testament and the New Testament. A testament of what? Of Jesus Christ as the Savior and Redeemer of the world. The Book of Mormon provides yet another testament, or witness, that Jesus is the Christ. It supports the other witnesses we already have, and helps us to better understand the full story. I don't know about you, but if I'm judging the case I'm going to want as many witnesses as I can get.
The Book of Mormon is not as radical as you may think. It is a beautiful record of God in our lives, in the lives of people that lived thousands of years ago who had faith in Christ's coming. I don't know about you, but I didn't walk and talk with Jesus Christ, and so I'm going on a lot of faith here. Because of that, I identify with these people on the American continent in a way I don't with others. They believe the words of the holy prophets that say a Messiah will come, and they have hope through that. I believe the words of holy prophets that the Messiah did come, and I have hope through that. When I read the Book of Mormon, I feel peace and I feel happiness - the same way I feel when I read the Bible. This book helps me to know God and know that He cares. I promise you if you read it and pray to God to know if it's true, He will answer you.
Words of Judah and of Ephraim will be joined together (Ezekiel 37:16-17)
Isaiah prophesies about a sealed book being read by one unlearned (Isaiah 29:11-14)
The Book of Mormon teaches of Christ (2 Nephi 25:26)
Introduction to the Book of Mormon
Testimony of Three Witnesses
Testimony of Eight Witnesses
"Safety for the Soul" (by Jeffrey R. Holland)