on Jane Friedman site:
Characters don’t exist in a vacuum: Who they are, what they want, and why they do what they do is rooted in who they have been and what they have done—in other words, backstory.
Backstory brings characters to life, gives them depth and dimension, and draws readers in. Without it characters may feel opaque or flat, their actions random or unmotivated.
But too much backstory can dilute and derail your actual story.
Backstory is a potent tool in your writing, and like all power tools it must be operated carefully—too much and your story may bog down and stall out; too little and readers may feel uninvested or confused. Finding that balance can be tricky.