As Stisa also points out in this post, there is a clear cultural context channeled by Moroni as he rends his garment, writes covenant language on it, and uses this symbol to unite his people to stand up for their defense. He doesn't stop there, either. Notice how in verse 23 he explicitly references Joseph in Egypt:
23 Moroni said unto them: Behold, we are a remnant of the seed of Jacob; yea, we are a remnant of the seed of Joseph, whose coat was rent by his brethren into many pieces; yea, and now behold, let us remember to keep the commandments of God, or our garments shall be rent by our brethren, and we be cast into prison, or be sold, or be slain.Notice in this verse the direct connection between the garment and faithfulness to the covenant.
I want to take this same idea and tie it into the concept of the covenant/garment themes laced (pun intended) throughout the Book of Mormon. In verse 21, the rending of garments becomes a ritual performed to remind the people of their covenant:
21 And it came to pass that when Moroni had proclaimed these words, behold, the people came running together with their armor girded about their loins, rending their garments in token, or as a covenant, that they would not forsake the Lord their God; or, in other words, if they should transgress the commandments of God, or fall into transgression, and be ashamed to take upon them the name of Christ, the Lord should rend them even as they had rent their garments.It goes without saying, then, that the people were already wearing garments to rend at the time of Moroni's impassioned speech. These Nephites, descendants of the tribes of Joseph, who were at the time living in the middle of the tropical jungle in Mesoamerica 500 years after Lehi left Jerusalem, were clothed in a garment of special significance.
For this to be the case so many generations later indicates to me how culturally vital it must have been to them. Remember that Joseph's "coat of many colors" (mentioned in the KJV in Genesis 37:3) is thought by some biblical scholars to be a mis-translation. Other scholars believe it refers to a ceremonial garment of special, temple-related significance:
"The article so described was a ceremonial robe which could be draped around statues of goddesses, and had various gold ornaments sewed onto it."Indeed, the NIV translates this as "ornate robe." The NRSV calls it "a long robe with sleeves." The Expanded Bible calls it "a special robe with long sleeves."
The Lamanites, on the other hand, were frequently described as naked. Alma 43:20, for example:
20 Now the army of Zerahemnah was not prepared with any such thing; they had only their swords and their cimeters, their bows and their arrows, their stones and their slings; and they were naked, save it were a skin which was girded about their loins; yea, all were naked, save it were the Zoramites and the Amalekites;Notice this reference mentioning that all of the Lamanites were naked except the Zoramites and Amalekites.
This begs the question: what is different about the Zoramites and the Amalekites that caused them to be clothed among all the naked Lamanites?
In short, both groups used to be Nephites. It seems that despite abandoning their loyalty to the Nephites, they maintain a cultural commonality and continued to wear a garment like the Nephites.