Usually I use these for my minor characters (mC), a non-lead, because those are the ones whom I am unsure about. Those are the characters I haven't spent enough time developing, and so I need to go the extra mile with them and make them real. I can't do that if I continue to ignore them to focus on my main character (MC). So a character bio is an invaluable tool for me.
What is a character bio?
I define a character bio as the protagonist's story from a mC's point-of-view. This may not be the "best" or the "correct" way to define it, but in keeping this definition in my mind, I can approach the mC as I need to. Although a mC's backstory will obviously influence them as much as a MC's backstory influences the MC, their story only concerns me in so much as it concerns the MC.
So with that definition in mind, I sit down to write my mC's bio. I'll begin where the MC begins--the beginning of the novel. I'll tell the story, trying to keep it brief, from the mC's POV, without worrying about whether I'm telling or showing or if this is coming out right. I'm only concerned with how this mC sees things. So I don't stop until I get to the end. This usually comes out as a far abbreviated version of the overall story--the mC can only tell the story as much as he/she knows it, and as he/she is a minor character, that's usually not much.
However, this tactic really allows me to make those minor characters much more real to myself and (hopefully) to the reader.
I always feel that a novel is at its most powerful when the mCs live and breathe as strongly as the MCs.