Character Bio's are important.
You have to know your character before you can write them. If you don't you'll end up writing a flat, cookie-cutter, person. You don't want that, do you?
Of course not!
So, how do we get to know these fictional characters we just pulled out of thin air?
We give them a life and history all their own.
And the simplest way to do this is to give them a resume. Pretend they are sitting down in front of you, applying to be a character in your story.
Start with the basics. (beyond the image stuff, which I assume you have already come up with; hair color, eye color, skin color, height, etc...)
What is their full name?
How old are they?
Where do they live?
How long have they lived there?
Did they ever attend school? If so, where? (I write vampires, so school could mean listening to the great Socrates, discus philosophy back in the A.D. days)
Any specialized degrees?
Previous work history? (the jobs we do help to shape us as much as our education. Did your character have to work some menial job they hated? Were they some high powered executive? For fantasy stories, rulers of Kingdoms and peasants count as well)
Now, once you have the basics out of the way. Ask your "applicant", what qualifies them to be a character in your story?
Are they applying for a main character or a secondary character position?
What special powers and/or abilities do they posses?
Ask them to list one good and one bad quality about themselves.
And finally, your character needs to provide references! What kind of friends do they have? What is their family like? Do they have any enemies that they wouldn't want listed on a resume? List a few of those people in their life, both good and bad.
If you (or your character) can answer all of these questions, you will be off to a great start. The goal is to have a character that not only feels real to you, but reads as a real person to your audience.
A background and history help to create that "real" feeling.