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 3 Nephi 26:11-12 Mormon states the following concerning the words of Christ to the Nephites:
Behold, I was about to write them all which were engraven upon the plates of Nephi, but the Lord forbade it, saying: I will try the faith of my people. Therefore I, Mormon, do write the things which have been commanded me of the Lord. And now I, Mormon, make an end of my sayings, and proceed to write the things which have been commanded 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]
3 Nephi 26:12 I, Mormon, do write the things which have been commanded me of the Lord ([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]