According to Raymond Treat, the principle, "What is in the Book of Mormon is There for a Purpose," is a very important principle to understand about the Book of Mormon. The dictionary tells us that a principle is a general truth on which other truths depend. The recognition of the "purpose principle" is the recognition of a general truth about the book of Mormon which in turn will lead to other truths. If we ask the question, "Why has this particular information been included?" every time we study a portion of the Book of Mormon we should receive insights that otherwise might be missed.
How do we know that what is in the book of Mormon is there for a purpose? Because major writers of the Book of Mormon tell us they were directed by God as to what to put in the Book of Mormon and what to leave out. For example, in Words of Mormon 1:6-9 Mormon states the following concerning the reason why he included the small plates of Nephi:
But behold, I shall take these plates, which contain these prophesyings and revelations, and put them with the remainder of my record, for they are choice unto me; and I know they will be choice unto my brethren. And I do this for a wise purpose; for thus it whispereth me, according to the workings of the Spirit of the Lord which is in me. And now, I do not know all things; but the Lord knoweth all things which are to come; wherefore, he worketh in me to do according to his will. And my prayer to God is concerning my brethren, that they may once again come to the knowledge of God, yea, the redemption of Christ; that they may once again be a delightsome people. And now I, Mormon, proceed to finish out my record, which I take from the plates of Nephi; and I make it according to the knowledge and the understanding which God has given me.
The chart "The Contents of the Book of Mormon Were Divinely Controlled" (see illustration) is designed to illustrate this point. The chart gives us information about three major Book of Mormon writers--Nephi, Mormon and Moroni. In each case these writers were told both what to put in the Book of Mormon and what to leave out.
For all practical purposes two of these three writers, Mormon and Moroni, controlled the contents of the entire Book of Mormon. Mormon was directed to add the entire contents of the small plates of Nephi to the Book of Mormon. . . . Nephi was responsible for about 82 percent of the contents of the small plates of Nephi, which strengthens the case even further that the contents of the Book of Mormon were indeed divinely controlled. They also give validity to the principle that what is in the Book of Mormon is there for a purpose. [Raymond C. Treat, "What Is in the Book of Mormon Is There for a Purpose," in Recent Book of Mormon Developments, Vol. 2, pp. 172-173] [See the commentary on 1 Nephi 19:3]
Words of Mormon 1:9 I make it according to the knowledge and the understanding which God has given me (Illustration): Chart: "The Contents of the Book of Mormon Were Divinely Controlled." [Raymond C. Treat, "What Is in the Book of Mormon Is There for a Purpose," in Recent Book of Mormon Developments, Vol. 2, p. 172]