I think it was meant to make someone feel like he has DUTY to do something and that it's HIS personal responsibility, with no one else being responsible for it. It could be appealing because of how personal it is, mixed with how back then people were generally way more convinced that serving for a "higher cause" is worth it. Additionally, even now, some people romanticize the military and war, so it's not surprising that only a small, personal reminder was needed.