Every single character I've ever written or seen or even read has always had to have a background. I can see a picture of a character, and immediately, I have to give it a story, whether it be a simple, "has had a good life" or a more devastating, "everything goes wrong". Every single character, including those that aren't mine, have to have their own story, their own motives and insecurities. For me, having a story, a background, is what gives them life.
I have noticed that it's our stories, our experiences that we as people use to connect and communicate with each other. And if that really is how we identify other people in our mind, by how they relate to you whether it be through looks, stories, or experiences, then getting a character to come to life means giving them something that others can relate or identify with, even if that connection is simply a reminder of someone they know in real life. After all, there's a reason why even aliens oftentimes have very human characteristics. And it's almost always a human or human-like creature that is the main character.
But that's just my two-cents...