Out of all the revelations given in the last days, the Book of Mormon is the most important. Many see the Book of Mormon as simply a rip off of Isaiah, Jesus' Sermon on the Mount, and Solomon Spaulding. However, there are so many intricate things within the book that continually reveals itself in new and important ways to us.
First, the Book of Mormon teaches us that God still speaks to mankind. The heavens are open! Revelation still occurs. This is a big part of the "new covenant" of the Book of Mormon - God speaking to man in all times and places.In Alma 29:8, we read that God gives to all nations and peoples the amount of truth they are ready to receive. But if people do not believe God can speak with them or send new prophets, then they severely limit the truth God wishes to give them.
Second, the Book of Mormon teaches us the "doctrine of Christ". In 2 Ne 31 and 3 Ne 11. The doctrine of Christ is that the Father, Son and Holy Ghost are "one God" or one in all things, and we are to learn to be one with them and each other. It is a process. We develop a level of faith in Christ. We recognize our sins and repent. We receive baptism or other ordinance, as it is through the ordinances that the "mysteries of godliness" are revealed (D"&C 84:19-21). Finally, we receive the Holy Ghost, which sanctifies us to a higher level of righteousness. Then we repeat the process, growing in faith and righteousness, until we become one with the Godhead.
Third, it warns us of secret combinations and the enemies of God. Those groups or organizations or people who seek to get gain by any means are the enemies of God. They can include secret signs and oaths, and involve getting gain by using violence or other criminal means. When such combinations take control of government, the righteous become the enemies of the state, or are punished for their testimony of Christ; while the wicked are viewed as popular and good by society.
Fourth, the Book of Mormon defines and shows the pathway to a personal theophany, or visitation of God to the individual. From Lehi's seeing God on his throne in 1 Nephi 1, to the Vision of the Tree of Life, to Alma's suffering in hell and then being redeemed into the presence of God's throne, to the Brother of Jared seeing the finger of God, we see the purpose of the Book of Mormon: to show us how to become like Christ, so we can return into the presence of God. Today, Latter-day Saints practice the theophany in the temple endowment, as the temple and Book of Mormon are closely linked by this new covenant and purpose.
There are other purposes to the Book of Mormon, but these are definitely the most important concepts. Without the Book of Mormon, there would be no second witness of Christ and his divinity. There would be no understanding of modern revelation. We would not understand fully the process to become one with the Godhead, and we would not understand the importance to enter into the presence of the Lord.