Believe me, you will be quite surprised after one week where extra calories in your diet might be appearing from. Another common scenario is the individual who eats a wonderful, balanced, healthy diet, but 'forgets' that cappuccinos, beer, wine, soda, etc. are all weight loss minefields, and consumes these indiscriminately. Writing down what you eat AND drink can pinpoint why you are not being as successful at losing weight as you would think.
A further reason is we all think that a cookie here or there, or the odd Mars bar, won't hurt us. True words indeed, but look at how many of these you consumed over the period of a week, and you might begin to understand that the 'odd' one has now become a little more regular than you thought.
Additionally, you will make smarter choices. You have to face your food diary, so instead of a 400 calorie ice cream, chances are if you have to write it down you have to think about it, so you will probably plump for the skinnier sorbet.
Food diaries also help you to see where your diet falls down nutritionally. You might think you eat plenty of fruit, but in reality it might be the same thing every day. By varying your fruit intake, you expose yourself to a greater array of vitamins and minerals, all necessary for good health.
Lastly, and most importantly, we all cram food into our mouth without much thought. Time has become our enemy, and is in scarce supply, so sometime its "the quicker the better" when it comes to food. We don't stop to think about what we are fueling our bodies with. We comfort eat when we are depressed, and eat well to celebrate success. Writing down what you eat really makes you think about how you are treating yourself. Seeing line after line of food that you know is not benefiting you can make you take a step back and assess your diet as a whole. It makes you aware of the empty calories that provide no nutritional value, and helps you to refocus and rethink how to improve your diet.
All of which can only mean positive progress for you and your diet.