Verse 12 is the conclusion of the string of arguments Nephi puts forth. It is more generic than the examples he has used, and serves for stylistic closure. It also provides a transition from the questions of the examples to the necessity of faith. Nephi's closure of the argument is heavily weighted to the experience of the plates. Not only is the appearance of the angel related to the plates, but the idea that the Lord can provide the means if we provide the faith is also a major lesson of the experience with the brass plates.
Nephi's exhortation to faith is somewhat unusual in the context of Old Testament literature. It is not that faith was unknown, but it is a seldom preached facet of religion in our current Old Testament texts. Nephi had learned a significant lesson about faith, and includes it in his conclusion.