Mormon did not know the “wise purpose,” but the Lord did. That purpose became plain when the first 116 pages of the translated abridgment were lost. The Lord explains to Joseph Smith during this episode:
And now, verily I say unto you, that an account of those things that you have written, which have gone out of your hands, is engraven upon the plates of Nephi;
Yea, and you remember it was said in those writings that a more particular account was given of these things upon the plates of Nephi.
And now, because the account which is engraven upon the plates of Nephi is more particular concerning the things which, in my wisdom, I would bring to the knowledge of the people in this account—
Therefore, you shall translate the engravings which are on the plates of Nephi, down even till you come to the reign of king Benjamin, or until you come to that which you have translated, which you have retained;
And behold, you shall publish it as the record of Nephi; and thus I will confound those who have altered my words.
I will not suffer that they shall destroy my work; yea, I will show unto them that my wisdom is greater than the cunning of the devil.
Behold, they have only got a part, or an abridgment of the account of Nephi.
Behold, there are many things engraven upon the plates of Nephi which do throw greater views upon my gospel; therefore, it is wisdom in me that you should translate this first part of the engravings of Nephi, and send forth in this work. (D&C 10:38–45)
In this revelation, the Lord makes no clear differentiation between the large and small plates. Just as Nephi gave them the same name, so does the Lord. Nevertheless, the two sets of plates must be what he means in verses 38 and 39: “And now, verily I say unto you, that an account of those things that you have written, which have gone out of your hands, is engraven upon the plates of Nephi; Yea, and you remember it was said in those writings that a more particular account was given of these things upon the plates of Nephi” (D&C 10:38–39).
Verse 38 clearly refers to the large plates—the source of the 116 manuscript pages “which have gone out of your hands.” Verse 39, however, shifts the emphasis. In the text of “those writings” (meaning the large plates alluded to in verse 38), the Lord notes that “it was said… that a more particular account was given of these things upon the plates of Nephi.” If we read “plates of Nephi” in this verse to be the same as the “plates of Nephi” in the previous verse, the document contradicts itself, stating that a more particular account was written in that same document—simultaneously a separate document, but the same. This would make no sense except that the small plates were also called by the same name and that they were physically included in the large-plate abridgment.
We may speculate, therefore, that at the end of the material immediately prior to the inclusion of the small plates, Mormon would have written a transition between the abridgment and the small plates similar to that in Words of Mormon. In that missing transition, the plates would have been named and described as a “more particular” account. Joseph Smith would have already read this transition and would remember it at the Lord’s reminder. It may be, however, that the Lord did not have him translate it at that time, again for the “wise purpose” of foiling a possible attempt to discredit the translation by altering the 116 manuscript pages.