That is the subtitle to this book of scripture, as discussed here. Another witness of Jesus Christ is valuable. See Doctrine and Covenants 6:28 and Matthew 18:16 as to why.
Consider the meaning of the word testament.
"In the Bible we have the Old Testament and the New Testament. The word testament is the English rendering of a Greek word that can also be translated as covenant. Is this what the Lord meant when He called the Book of Mormon the 'new covenant'? It is indeed another testament or witness of Jesus. This is one of the reasons why we have recently [in 1982] added the words 'Another Testament of Jesus Christ' to the title of the Book of Mormon." 
It was in 1982 when the leaders of the Church decided on this subtitle. Suddenly, the stick (or record) of Judah - the Old Testament and the New Testament - and the stick (or record) of Ephraim - the Book of Mormon, which is another testament of Jesus Christ - were "woven together in such a way that as you pore over one you are drawn to the other; as you learn from one you are enlightened by the other. They are indeed one in our hands. Ezekiel's prophecy now stands fulfilled." 
This is an overview of the ancient records within the Book of Mormon:
In the second paragraph of the Book of Mormon's title page, we read what the Book of Mormon writers wanted to convince the world ... the purpose of the Book of Mormon is to bring Jew and Gentile to the conviction that "Jesus is the Christ, the Eternal God, manifesting himself unto all nations."
(By the way, the word Christ means "anointed one" in Greek. In Hebrew, Messiah means "anointed one". In 1 Nephi 10:4, the prophet Lehi referred to Jesus Christ as "a Messiah, or in other words, a Savior of the world". Therefore, when we say that Jesus is the Christ, we are testifying that He was the One who was anointed to save us.)
How has the Book of Mormon convinced you that Jesus is the Christ?
Can you think of specific passages from the Book of Mormon that have strengthened your testimony of the Saviour?
I think of the powerful conversion story of King Lamoni, in the book of Alma. Once he was taught about God and Jesus Christ, his Saviour, Lamoni felt so moved to give up all his sins and way of life to know the truth. Read the account here.
 Ezra Taft Benson, in Conference Report, Oct. 1986, 4; or Ensign, Nov. 1986, 4
 Boyd K. Packer, in Conference Report, Oct. 1982, 75; or Ensign, Nov. 1982, 53
 History of the Church, 1:71