How important were angels in the history of the Nephite world?
The same angel that had converted Alma and the four sons of Mosiah directed Alma to return to Ammonihah to deal with the Nehorites (8:15). Whether the same angel came to guide Amulek to aid Alma and to provide a second witness for him, we do not know, but it seems that it might have been. In any event, where would the Nephite world be without those messengers? If the Nehorite movement had been able to gather momentum, what would have happened in Zarahemla? And where would the Restoration of the Gospel by Joseph Smith have stood if it had not been for numerous angelic visitations?
Many of us may not have had angels visible in our lives, but angels are present more often than we realize. Elder Bednar called these types of events "tender mercies." President Eyring has asked members of the Church to write down these mercies as they occur in their lives. Watch for such events; they do happen.
Book of Mormon Central, "Why is Amulek’s Household Significant?" KnoWhy 117 (June 8, 2016).
For a compilation of the visitations received by Joseph Smith, see Alexander L. Baugh, "Parting the Veil: Joseph Smith’s Seventy-six Documented Visionary Experiences," in Opening the Heavens, ed. John W. Welch (Salt Lake City and Provo, UT: Deseret Book and BYU Studies, 2005), 268–273, 280–289.