Writing down every single thing you eat (including what you scarf down at the kitchen counter after dinner) helps keep your accountability for your food choices front and center. You can’t pretend you’re on track if you’re not.
Research has proven it works. In a California weight loss study, half the subjects were asked to write down everything they ate. By the end of the study, those who kept food diaries lost more weight. Mind you, they were not told what to eat, how much to eat, or how often. That was up to them. It was simply a matter of increasing their commitment to themselves by putting it all in black and white.