"Yea, come unto Christ, and be perfected in him, and deny yourselves of all ungodliness; and if ye shall deny yourselves of all ungodliness, and love God with all your might, mind and strength, then is his grace sufficient for you, that by his grace ye may be perfect in Christ; and if by the grace of God ye are perfect in Christ, ye can in nowise deny the power of God."
Moroni 10:32
Here as Moroni concludes the Book of Mormon he summarizes the central purpose for the book. The purpose of the Book of Mormon is to bring us (the readers) to Christ. Why? So that we can ultimately be saved by returning to the presence of God.
The Book of Mormon's title page also lists 3 purposes for the Book of Mormon, including the central purpose I mentioned above:
1. "To show unto the remnant of the House of Israel what great things the Lord hath done for their fathers."
2. "That they [the House of Israel] may know the covenants of the Lord, that they are not cast off forever."
3. For "the convincing of the Jew and Gentile that Jesus is the Christ, the Eternal God, manifesting himself unto all nations."
Book of Mormon Title Page
So, once again we see that the central purpose is to bring us to our Savior, Jesus Christ! What a wonderful goal. I know that this book has greatly helped me come to know my Savior and trust in Him.
I found a lot more information about the Book of Mormon here . Usually the best place to get information is from the source, so that link (from the Mormon (LDS) Church is a good source).