As a fiction writer, you have to know your character’s back story (what has happened in your character’s life before your story begins) and you need to carry it with you as you write. If you don’t, you lose authenticity.
Let’s say your main character’s mother drowned years ago and, at some point in your novel, your character goes on a boat ride. Well, if your character never once thinks about her mom while on that boat ride, then something will probably feel false or missing about the scene to the reader even if the reader isn’t conscious about what’s missing.