Mormon did not know the precise nature of the “wise purpose.” Certainly the Lord did. That wise purpose became apparent when the 116 pages of the translation of Mormon’s abridgment was lost. The Lord explains this to Joseph Smith after his ability to translate was lost for a season:
D&C 10:38-45
38 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;
39 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.
40 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—
41 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;
42 And behold, you shall publish it as the record of Nephi; and thus I will confound those who have altered my words.
43 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.
44 Behold, they have only got a part, or an abridgment of the account of Nephi.
45 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.
Most fascinating about these verses is that there is no clear division between the large and small plates of Nephi. Just as Nephi called them the same thing, so the Lord appears to make no distinction. Nevertheless, the two sets of plates must be referenced here. Note the enigmatic reading of verses 38 and 39:
D&C 10:38-39
38 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;
39 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.
In verse 38 the very clear reference is to the large plates, because the reference is to the source of the 116 manuscript pates “which have gone out of your hands.” Therefore in verse 38 “plates of Nephi” must refer to the large plates.
In verse 39, however, there is a shift in emphasis. In the text of “those writings” (forcibly referring to the previous verse, and therefore the large plates) 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, we are left with the contradictory idea that the document stated that there was a more particular account written in that same document. At one and the same time it would suggest a separate document, but hold that they were the same document. This makes no sense except that the small plates were also called by the same name, and that they were physically included in the large plate tradition.
We may speculate, therefore, that at the end of the material immediately prior to the inclusion of the small plates, there was a similar transition between abridgment and small plates as we have in Words of Mormon. In that missing transition, the plates would have been named, and described as a “more particular” account. Joseph would have already read this transition in order to have the Lord call it to his remembrance. It may be, however, that the Lord did not have him translate it at that time, again for the “wise purpose” of foiling the possible attempt at discrediting the translation through the alteration of the 116 manuscript pages, which was Joseph’s fear.