As an author I think it's important to know the history of your characters. It's what gives them depth. I think most authors do a good job of showing the physical things about a character. You know, hair color, height, eye color, rock hard abs. Oops. How did that slip in there? *grins*
I think the emotional past is much much harder to create. Think about your own life. You have things in your past that not very many people know about. Things that make you who you are. Things that make you more sensitive to specific situations, or things that make you more empathetic, or things that help you "get" others. These "things" drive how you act, how you react, and how you live your life.
In order to create believable, three-dimensional characters, they must have this past too. And you must know it. Not only that, you must be able to convey it in the writing. Their decisions should be consistent with their past. Their reactions must be believable and authentic.
Without a background check, the characters may be hawt, but not much else. And since we've all been watching Dating In The Dark (you know you watched. Spill...spill...), we know that sometimes how someone looks isn't the deciding factor. Sometimes it's what they do. Don't do. Say. Don't say. Believe. Don't believe. It's WHO they are.
So in your writing, make sure you know who your characters are. Do that background check.
What do you think? Do you fingerprint your characters? How do you log their past? Me? I keep everything really nice and tidy in this huge filing cabinet called my brain. ROTFL. What about you?