In addition to reiterating the commandment to leave Jerusalem, Nephi expounds on the reasons for obtaining the plates. He speaks of preserving their heritage and scriptures for their children. It is not known whether or not this reason was given him by Lehi, or whether he deduced it for himself. Clearly Nephi was sufficiently intelligent to come with the information on his own. Nephi's reasons have validity only when we understand that Nephi knew that their descendants were to be cut off from their previous culture. Certainly Laman and Lemuel would have thought that they would "lie low" for a while and then return to their own people. Nephi's argument that the plates were essential to their cultural preservation indicates his understanding that this journey was one which would completely and inalterably sever the physical ties with their land and culture, thus the need to preserve the scriptural heritage.
Textual analysis: In verse 19 Nephi notes that they must obtain the plates to "preserve unto our children the language of our fathers". This might be seen in contrast to the Mulekites who brought no such records: "...and their language had become corrupted; and they had brought no records with them; and they denied the being of their Creator; and Mosiah, nor the people of Mosiah, could understand them" (Omni 1:17).
While the Omni citation for the Mulekites clearly indicates that their speech was corrupted, it is not clear that the meaning of the word "language" in either verse relates to our modern definition of the spoken word. As indicated in the discussion of 1 Nephi 1:2, "language" is frequently used in the Book of Mormon to indicate a body of learning, a cultural heritage, rather than grammar and vocabulary. Nephi's desire to preserve their "language" is much more likely to be related to the cultural aspects, since the brass plates were apparently written in Reformed Egyptian, not Hebrew, and therefore would be less likely to preserve grammar and vocabulary.
As for the usage in Omni, it is ambiguous. Note that right after discussing the corruption of "language", the examples given are that they denied their creator. The inability of Mosiah and his people to understand them comes in the next phrase. While certainly not conclusive, the suggestion is that the corruption that most worried them was the loss of their heritage and culture. Note that all of this information comes after they were able to sufficiently understand them that they could know of their ultimate genealogy.