Writing is a full time job. Not only do writers live their own lives, but the lives of countless others, too. For every character we write, we have to know what makes the tick. And, to do that, we must know their experiences and pasts so we can create their reactions in their stories. Even if you may never have the opportunity to use it, it can be good practice to write out, in whatever style you’re working with, major events from your character’s lives. This way, you know exactly how they reacted to them, what impressions they got from the events and the exact ways it changed them.
The story the reader will eventually get is only the surface of what the writer must know.