Know thy self, was written in a temple in Delphi in the novel "Alexander:child of a dream",something which is truely important. Knowing one's self doesn't only define your life, it also defines other people's lives. When someone knows who he/she is in the world, it's easier to react to other people, to establish boundries and founding other people's personalities.
I believe that a personality grows through "action and reaction", which is simply called experience.
Yet this is not the only benefit, another is that no one can ever make you feel small, or unimportant, not because of vanity, but of certainity. Doubt can never sneak in a life that's detailed and organised. Even if it did, it wouldn't find a place to stay.
So, understand, live and react. The rest will come naturally.