Work changed the Lamanites
Now Moroni was compelled to cause the Lamanites to labor, because it was easy to guard them while at their labor.
To us this may seem like no big deal. After all, in our modern day the Geneva Convention makes clear that this is perfectly fine as long as the labor falls into the classifications set forth in Article 50. But the fact that Moroni mentions it makes it sound like turning your prisoners into workhorses maybe was not the norm in warfare at that time. Furthermore, this concept of daily, taxing work was probably somewhat of a foreign concept to the Lamanites.
The Lamanites were an idle people
From the very beginning, Nephi beheld in his vision that the seed of his older brothers would be "a filthy people, full of idleness" (1 Nephi 12:23). Just a few years after his vision, he sadly observed the realization of that prophecy: "And because of their cursing which was upon them they did become an idle people" (2 Nephi 5:24).
Here's a further illustration of this point. The Nephites built a largely agrarian society. Enos records "the people of Nephi did till the land, and raise all manner of grain, and of fruit, and flocks of herds, and flocks of all manner of cattle of every kind, and goats, and wild goats, and also many horses" (Enos 1:21). They were skilled in plowing the earth and working for their sustenance according to the law of the harvest.
Contrast that to the Lamanites who "became wild, and ferocious, and a bloodthirsty people, full of idolatry and filthiness; feeding upon beasts of prey; dwelling in tents, and wandering about in the wilderness with a short skin girdle about their loins and their heads shaven; and their skill was in the bow, and in the cimeter, and the ax. And many of them did eat nothing save it was raw meat" (v. 20). Rather than adopt the agricultural economy of the Nephites, the Lamanites built their diets and their culture primarily on the meat of animals they could kill in one day and embraced filthiness, and nakedness.
Mormon said it most succintly when he described the Lamanites as:
A wild and a hardened and a ferocious people; a people who delighted in murdering the Nephites, and robbing and plundering them; and their hearts were set upon riches, or upon gold and silver, and precious stones; yet they sought to obtain these things by murdering and plundering, that they might not labor for them with their own hands.
This was not globally true of all Lamanites (Alma 22:28 tells us that some Lamanites were more idle than others), but as a general rule, the Lamanites had maintained a centuries-long tradition of avoiding work. And when Ammon and friends brought Christianity to them, the converts who buried their weapons of war were not only doing it as a sign of their unwillingness to shed blood, but also as "a testimony to God, and also to men, that… rather than spend their days in idleness they would labor abundantly with their hands" (Alma 24:18). The distinction between the hard-working nature of the Nephites and the idleness of the Lamanites was a sharp one.
So now the Lamanites are presumably in a new circumstance. They have to work all the time. It's back-breaking, manual labor, too. What effect does this have? First, as noted above, the work keeps the Lamanites easy to guard. Was it just that they were too distracted with their tasks to plan a rebellion against their Nephite taskmasters? I think it was more than that. I think it was the work itself working a change on them.
Work opens us up to accomplishment
Imagine yourself in the position of a Lamanite prisoner for a moment. Like many in society today, you were told ever since you were born that you and those like you are victims. Victims of a crime committed centuries ago by the ancestors of the wealthy, hard-working and more successful fair-skinned "others." You live your whole life without really building anything, accomplishing anything, or doing anything constructive. And even though your people hate the others, every time any of them dissent over to your side they always manage to take control of your armies and start another war that your people always end up regretting. They always come out ahead, and now you are in prison after your army fell for a simple decoy manuever.
But now you're working like you probably never worked in your life. It's grueling, it's tedious, but you're accomplishing something. You're digging a pretty impressive ditch. You're piling a huge mountain of dirt. You're learning to construct a huge wooden wall with pickets at the top "to an exceeding height." It's work, but it's good, quality work. So good that a wartime historian centuries later would record that, "this city [Bountiful] became an exceeding stronghold ever after" (Alma 53:4-5. So strong that this prison/city that persecuted Christians would gather to it in a few decades. So stong that it would be one of the cities to survive the great destruction at the Savior's death a few decades later. In fact, it is where the resurrected Son of God would descend to the Earth to visit his other lost sheep.
Even without knowing that last part, how would that make you feel? Yes, technically you're building your own prison, but you just built an entire city/fort! You learned construction, you learned how to work materials, and how to buid something. I believe the Lamanites rightly took some pride in what they had been able to accomplish, and probably got their first taste of the joy of creation and improvement.
Work keeps Satan from distracting us
You know those moments when you are so involved in something you're working on (like a Book of Mormon blog) that you don't even notice the passage of time? I think the Lamanites got their first taste of "being in the zone" like that. I am reminded of the story Pres. Uchtdorf shared about Nehemiah and his people rebuilding the walls of Jerusalem. His people "had a mind to work." They labored arduously day after day. When they were entreated by evil-seeking men, Nehemiah and his people were not distracted. He responded to the adversary each time by saying "I am doing a great work, so that I cannot come down." When we are so thoroughly involved in a great work, we cannot be easily distracted by the temptations of Satan to be slothful, to be covetous, or to dwell on improper thoughts.
Work exposes us to hard workers
The Lamanite prisoners spent at least a few years in the prison/city/work yard of Bountiful. And throughout that time, they surely had regular interactions with their Nephite guards. And over time they came to realize that their conceptions of the Nephites were not true at all (much like those who are antagonistic towards the Church soften their attitudes when they have LDS friends or coworkers). The example of the Nephites touched their hearts. Towards the end of the war, they ask to enter into a covenant of peace. Every single Lamanite prisoner joins the Christian converts of Ammon, to live out the rest of his life peacefully with the Nephites (62:27-29).
Work is power
There is a power in the nature of good hard work. I am blessed to have had a father instill that value above all others from a young age, and I have seen many blessings from it. But even as a Church, we are blessed with leaders who teach the same principle in word and example. As Pres. Monson says, "Work Will Win When Wishy Washy Wishing Won't." This is especially true when it comes to changing lives. As a missionary I stood with my districts and zones and repeated every week this quote from President Benson about the effect of good hard work on a missionary's attitude and soul:
One of the greatest secrets of missionary work is work! If a missionary works, he will get the Spirit; if he gets the Spirit, he will teach by the Spirit; and if he teaches by the Spirit, he will touch the hearts of the people and he will be happy. There will be no homesickness, no worrying about families, for all time and talents and interests are centered on the work of the ministry. Work, work, work– there is no satisfactory substitute, especially in missionary work.
What work are you involved in that is changing your life?