Why do people need to put character flaws in stories? Because characters are more believable that way. If you don't believe me, take a look at this article on Wikipedia on the all-important Character Flaw. At one point or another I have borrowed a few of these character flaws so that people will relate to my heroes and villains. You don't need many. One or two should do.
Derek from Mr. Dead Eyes once had a fear of dead bodies, of death itself. His friend Kurt has Claustrophobia which plays a huge role in breaking his calm exterior.
Another one of my characters has a fear of heights, another a fear of guns.
Chances are, you've read about similar characters. They represent a common theme; if you're going to write characters...you have to first make them human.