Scriptures were very important to the people whose history is recorded in the Book of Mormon. At the beginning of the Book of Mormon narrative, a Jerusalem prophet named Lehi is warned of the impending destruction of the city (around 600 B.C., just prior to the Babylonian captivity). His son Nephi recorded,
For it came to pass in the commencement of the first year of the reign of Zedekiah, king of Judah, (my father, Lehi, having dwelt at Jerusalem in all his days); and in that same year there came many prophets, prophesying unto the people that they must repent, or the great city Jerusalem must be destroyed.
Therefore, I would that ye should know, that after the Lord had shown so many marvelous things unto my father, Lehi, yea, concerning the destruction of Jerusalem, behold he went forth among the people, and began to prophesy and to declare unto them concerning the things which he had both seen and heard. And it came to pass that the Jews did mock him because of the things which he testified of them; for he truly testified of their wickedness and their abominations; and he testified that the things which he saw and heard, and also the things which he read in the book [which had been shown to him in a vision], manifested plainly of the coming of a Messiah, and also the redemption of the world.
And when the Jews heard these things they were angry with him; yea, even as with the prophets of old, whom they had cast out, and stoned, and slain; and they also sought his life, that they might take it away. But behold, I, Nephi, will show unto you that the tender mercies of the Lord are over all those whom he hath chosen, because of their faith, to make them mighty even unto the power of deliverance (1 Nephi 1:4, 18-20).
The Lord extended His mercy to Lehi and his family by leading them away from danger. He vowed to lead them to a “promised land” in order to preserve a righteous branch of Israel. The family, with some friends, left behind their ample possessions and journeyed three days into the wilderness. It was there that the Lord revealed to Lehi that he should send his sons back to Jerusalem to recover a scriptural record kept at the house of Laban:
For behold, Laban hath the record of the Jews and also a genealogy of my forefathers, and they are engraven upon plates of brass. Wherefore, the Lord hath commanded me that thou and thy brothers should go unto the house of Laban, and seek the records, and bring them down hither into the wilderness.
The brothers did so at great risk to their personal safety. The risk was worth it because of the value of the scriptural record:
And after they had given thanks unto the God of Israel, my father, Lehi, took the records which were engraven upon the plates of brass, and he did search them from the beginning. And he beheld that they did contain the five books of Moses, which gave an account of the creation of the world, and also of Adam and Eve, who were our first parents; And also a record of the Jews from the beginning, even down to the commencement of the reign of Zedekiah, king of Judah;
It reveals the “Jewishness” of the Book of Mormon peoples. Their new additions to their body of scripture are often written in chiasmus (Jewish scripture poetic form) and contain many Hebraisms, continuing the style of the scriptures already in their possession. Nephi constantly refers back to the story of the Exodus when he attempts to counsel his wayward brothers.
Therefore let us go up; let us be strong like unto Moses; for he truly spake unto the waters of the Red Sea and they divided hither and thither, and our fathers came through, out of captivity, on dry ground, and the armies of Pharaoh did follow and were drowned in the waters of the Red Sea. Now behold ye know that this is true; and ye also know that an angel hath spoken unto you; wherefore can ye doubt? Let us go up; the Lord is able to deliver us, even as our fathers, and to destroy Laban, even as the Egyptians (1 Nephi 4: 2, 3).
The Book of Mormon refers to Old Testament prophets whose writings are no longer in the canon, helping us to realize that the existing scriptures are incomplete. (See Lost Books.)
The Book of Mormon refers to many Old Testament events and people that some current scholars consider myths. For instance, there is a reference to Job, to the story of Jonah, to the creation and Adam and Eve, to the identity of Melchizedek, to the parting of the Red Sea by Moses, to the tower of Babel and the confounding of languages, to the flood, and to the greatness of King David. These references validate the biblical account.
The Book of Mormon includes some sections of Isaiah that refer to the dispersion and gathering of Israel. These sections directly pertain to this branch of Israelites led away to become the Book of Mormon people. It was a comfort to them to know that their descendants would be regathered. When Joseph Smith translated these sections, his translation ended up containing elements from various Isaiah translations available at the time, but not available to him. This adds to the remarkable nature of the Book of Mormon. Joseph Smith had only three years of elementary school education, and no library of materials to help him. These sections of Isaiah are related to additional Book of Mormon scriptures that enlighten the Isaiah writings to our understanding.
The Book of Mormon includes quotations from biblical sources that scholars claim to be late additions to the biblical record, validating their ancient origin.
Is it a bad thing that the Book of Mormon quotes or upholds the Bible?
“…any biblical scholar knows that it would be extremely suspicious if a book purporting to be the product of a society of pious emigrants from Jerusalem in ancient times did not quote the Bible. No lengthy religious writing of the Hebrews could conceivably be genuine if it was not full of scriptural quotations (Dr. Hugh Nibley).
Some are bothered that the sections quoting Isaiah are similar to the King James Version, and parts of the Book of Mormon written after the resurrection of Christ are similar to the New Testament KJV.
“…why should anyone quoting the Bible to American readers of 1830 not follow the only version of the Bible known to them? Actually the Bible passages quoted in the Book of Mormon often differ from the King James Version, but where the latter is correct there is every reason why it should be followed. When Jesus and the Apostles and, for that matter, the Angel Gabriel quote the scriptures in the New Testament, do they recite from some mysterious Urtext? Do they quote the prophets of old in the ultimate original? Do they give their own inspired translations? No, they do not. They quote the Septuagint, a Greek version of the Old Testament prepared in the third century B.C. Why so? Because that happened to be the received standard version of the Bible accepted by the readers of the Greek New Testament. When “holy men of God” quote the scriptures it is always in the received standard version of the people they are addressing.” 
Read more about the Brass Plates.
Read more about perceived “anachronisms” in the Book of Mormon.