Determining how much and what parts of your backstory to include are crucial to achieving the proper balance of it in your story.
Backstory influences plot events, character motives, and thematic subtext. Here are the only 4 questions you need to find your best backstory.
Source: 4 Ways to Write Backstory That Matters (How to Outline for NaNoWriMo, Pt. 5) – Helping Writers Become Authors