The helmet protects the head, the part of the body most vital to quality of life. It is the thinking part where choices and judgments are made, where attitudes reside and surge forth in conduct. It is the part that holds knowledge, understanding, wisdom, and memories of life's experiences, that determines the kind of life we lead. It is that part where Satan aims most of his fiery darts.
In this metaphor, hope is not an offensive weapon but a defense; it is a motivator to protect us from losing sight of the glorious end of God's purpose. Why? The only thing that can really defeat and destroy us is for us to give up. Jesus says in His Olivet prophecy, "But he who endures to the end shall be saved" (Matthew 24:13). Hope's fruit is not just an optimistic and positive outlook but also the drive to persevere, to endure come what may, to propel one forward. Only the hopeful will do this. The hopeless will give up.
So powerful is hope's action that Paul says in Romans 8:24-25 that we are saved by it! "For we are saved in this hope, but hope that is seen is not hope; for why does one still hope for what he sees? But if we hope for what we do not see, we eagerly wait for it with perseverance." This in no way conflicts with his declaration in Ephesians 2:8 that we are "saved by grace through faith," because both faith and hope are necessary for salvation. Faith primarily operates in the present as visible evidence of things hoped for but not yet seen. Hope, though it is also operating in the present, primarily does so with reference to the future. Paul then says that, if a person has hope, he is motivated to wait patiently for what he hopes to see. The hopeful are motivated to endure whatever it takes to receive what they hope for.