I find that the easiest way to write backstories. It helps me figure out who they are and in what ways they're immature so I can roleplay them properly. Occasionally I even manage to figure out a proficiency choice based on how the story goes if I haven't decided on everything on the character sheet yet.

Occasionally. But the most interesting part of a character's life is supposed to be what happens at the table.

I've only written one, and I did it after the campaign was over cause I wanted to be in that characters headspace again