Most of what follows was inspired by thoughts provided in Brant A. Gardner’s Second Witness series.
At the end of 1 Nephi chapter 2, Nephi has had a vision that shows him that he will become a ruler and a leader over his brethren. Nephi continually makes parallels between his and Joseph in Egypt’s life to help support the unusual calling for a younger brother to rule over his siblings. His vision came as a result of “being grieved because of the hardness of [his brothers’] hearts” and so he “[cried] unto the Lord.”
Chapter 3 then starts with him excited to tell Lehi about the vision and ensuing comfort received, but Lehi speaks first. Interestingly, Lehi has also had a vision. We’re not told about what prompted Lehi’s praying or if the vision came on it’s own. I’d like to think that Lehi, as a parent, was aware of his son’s concerns about them leaving Jerusalem. He likely was aware of Nephi’s attempt to soften their hearts and provide a second witness to the truthfulness of their father’s words. Naturally, this seems like something Lehi would be praying to the Lord about.
The directive to retrieve the brass plates seems like a perfect answer to what Nephi and possibly Lehi were praying about. The plates, we are told, were necessary so that they and their posterity would not “dwindle in unbelief.”
The Lord doesn’t always give us everything we need to start off with. He didn’t provide the plates, liahona, sword of Laban, etc. at the outset of their journey to the promised land. He waited for the opportune time to provide each, emphasizing important principles to them and the millions of people who have since read their words.