Nobody's perfect; everybody has limitations, imperfections, problems, phobias, or deficiencies that can hamper them. That's all part of being sentient. Character traits are a way of representing a character's weaknesses without having a major impact on the game, in order to add depth and aid the player in roleplaying the character. As with a feat, a character must fulfil the prerequisites for the flaw in order to select it, so a character cannot select the poor liar trait if they are trained in the bluff skill.