Why should you keep a log of your daily nutrition and workouts?

"I can't emphasize enough how important it is to write down your diet program and keep track of exaclty what and how much you are eating on a daily basis." - Arnold Schwarzenegger, The New Encyclopedia of Modern Bodybuilding

“Keep track of each and every set and rep you do. This may seem a bit tedious at first, but it literally takes 3-5 seconds to jot it down after each set. Besides, your training log will prove to be worth its weight in gold!” - Dr. Clay Hyght, 9 Things I Wish I Knew When I Started Bodybuilding

"Start keeping a training journal. Your numbers should go up... If they don't, that's a problem" - Dan John