There is a reason that most fanfiction authors, specifically girls, start with a Mary Sue. It’s because girls are taught that they are never enough. You can’t be too loud, too quiet, too smart. You can’t ask too many questions or know too many answers. No one is flocking to you for advice. Then something wonderful happens. The girl who was told she’s stupid finds out that she can be a better wizard than Albus Dumbledore. And that is something very important. Terrible at sports? You’re a warrior who does backflips and Legolas thinks you’re THE BEST. No friends? You get a standing ovation from Han Solo and the entire Rebel Alliance when you crash-land safely on Hoth after blowing up the Super Double Death Star. It’s all about you. Everyone in your favorite universe is TOTALLY ALL ABOUT YOU.
I started writing fanfiction the way most girls did, by re-inventing themselves.
Mary Sues exist because children who are told they’re nothing want to be everything.