Alma 17:16

therefore this was the cause for which the sons of Mosiah had undertaken the work, that perhaps they might bring them unto repentance; that perhaps they might bring them to know of the plan of redemption;

This is why the sons of Mosiah started this work. Maybe they could help them change their hearts. Maybe they could help them learn about God’s plan to save them.

That’s why the sons of Mosiah took on this work: to bring them to repentance, to help them understand God’s plan of redemption.