What makes a hero a hero and why is your character so important?

Often times in games, when you start your told ok your the hero go save the world, go save the galaxy. But your never really told why. What is it that makes you so important to the story.
Thinking back to mass effect 1, when you start the game as Commander Shepard, your already the hero. XO of the Normandy, high ranking marine and the crew of the Normandy already respect you. You can chose a backstory sure but you never get to experience any of it. The first steps of the journey.
Other games like Dragon Age Origins and Jedi Knight Jedi Academy give you the opportunity to start from the beginning and earn the right to become important and become the hero.
Seeing a characters journey from nobody to hero is definitely something I hope to see more of in Rpg games.

I guess its just ingrained in the human pysche to want to do good especially for the benefit of others. Thats why even in video games and movies you have various characters going on epic adventures doing heroic deeds.