A.B. Simpson emphasizes the importance of those who 'stay by the stuff' in the context of David's victory over the Amalekites, illustrating that both the front-line soldiers and the rear guard share equally in the spoils. He argues that the home guard is just as vital as those who go into battle, and that God calls some to support missions from home, contributing through prayer, finances, and encouragement. Simpson encourages believers to embrace the missionary idea, propagate it, and actively support those who are sent abroad, highlighting that every effort counts in the mission of evangelizing the world. He calls for a collective commitment to prayer and support for missionaries, asserting that the work at home is equally significant in advancing God's kingdom.