The other night, I wrote the back story to the back story of my book. An interesting concept. It got me excited. What I wrote the other night won't appear in my book in its current form. But by knowing what happened at the beginning of the history that impacts my story, I know where I will be going with my story. Great fiction writers spend a lot of time developing the back story...where it all started, what happened in their characters' histories, what happened just before the story begins, etc. Most of this doesn't even appear in the book. However, all this solid back story impacts what happens in the book with the characters. It makes a lot of sense to make sure you solidly know where you're coming from.
To put it in the light of today's cinema, even Hollywood has the Man of Steel himself ask the age old question about his back story.
In real life, it is the same. At some point in life a person will ask these thought-provoking questions: Where did I come from? Why am I here? Where am I going? If a person knows where they come from, it gives them purpose to what they are experiencing. Knowing where they can end up impacts their choices. My book's back story affects my characters' choices and future. My back story impacts my choices and my future.