Why are everybody nagging about keeping a food journal?

Knowing when and why you eat is an important step in recovery. Therefore it's often recomended in self help litterature (about eating disorders) to keep a daily journal of your eating habits. List what you ate, when you ate, your hunger level, and your feelings before and after you ate. This is not suppose to be an activity in order to judge yourself. Don't feel guilty. Instead be like a detective eager to reveal a secret. It sounds silly but hey, I'm not the one writing the self-help books. However I've tried this before and believe me it works.