First watch this:
Second: It's about the message being sent. Big Boss is The Legendary Soldier. He's a charismatic hero to the people around the world, his words, and his actions, reach far and wide across the planet. If he chooses to willingly, without any kind of outside pressure to denuclearize, word of that will reach other people and their governments around the world. People will point to The Diamond Dogs, at the Private Military Company, headed by one of if not THE greatest soldier ever born, and that they willing chose to disarm the weapon of mass destruction that would otherwise have been used to make them World Power. It's about The Hero choosing The Path Of Peace. Is there some concrete mechanical reward for doing so? Probably not but it's not about the mechanical reward. It's about the story, the message, of Big Boss choosing to be better.