A.B. Simpson emphasizes the importance of being 'adjusted' in every good work as expressed in Hebrews 13:20-21. He explains that true adjustment involves aligning ourselves with God's will and the circumstances around us, rather than trying to change them to fit our desires. This adjustment leads to spiritual rest, power, and freedom, allowing us to fulfill God's plan for our lives. Simpson encourages believers to seek God's fullness and to be used by Him in every aspect of life, ultimately leading to joy and glory in His presence.