Nephi's reference to the land of promise is a direct reiteration of the direction he was given in a personal revelation recorded in 1 Nephi 2:20. It is interesting to speculate on what Lehi's family knew of the ultimate direction of their journey.
When Lehi receives the command to leave Jerusalem, it appears as a command to leave Jerusalem and any indication that there was an ultimate destination rather than a simple escape is not written (see 1 Ne 2:1-3). While Lehi may have known that they were heading toward a land of promise, this is not explicitly stated in the text until the information comes from Nephi's revelation. If this distinction were correct, it is yet another indication that Lehi was the prophet for the Old World and the exodus from Jerusalem, and Nephi was the prophet for the New World, with the revelations he received looking forward to that time and location.
In any case, until this point in the narrative, there must have been an assumption that the land of promise would provide the necessities of life - and (had they even thought of it by then) wives for Lehi's sons. This revelation clarifies that wives would be a necessity, and the Lord requires that they return for them. There are two interesting facets of this command to return. Why would they need wives, and why now?
The "why" of the wives would be obvious if there were no women where they were going. However, there was no known part of the world at that time period where women would have been non-existent. The complete lack of availability of women could not be the case. What other factors might have required them to return for women?
The first is that even though women might be available through other peoples in other lands, the access to such women would be difficult. There would be difficulties in culture, perhaps language (since they did not know yet where they were going) and most importantly religion. The marital laws of Israel would have required marriage within the religion, and leaving the land of their first inheritance they would clearly be leaving the ability to comply.
Why now? Certainly they needed to acquire wives before it was too difficult to return to obtain them. Why after the foray for the plates, and not before? Or not at the same time? While the scriptures offer no answer, it is probable that the experiences of acquiring the brass plates solidified the reality of the exodus in the minds of the family, and firmly cut the ties of possible return. Once the sacred plates were obtained, they were committed to the journey, and they were also more committed to the idea that the Lord was leading them. Thus they had those experiences to strengthen their resolve, and perhaps to use to convince Ishmael that he should accompany them.