Many people who see dietitians need more than information about what is wrong with their diet. They often need to solve problems in eating behavior. Getting all the facts down about that behavior can really help to identify problems. Regardless of how much information you choose to log, chances are good that you'll learn something from your food diary. But you also might find that some of the vegetarian convenience items you are eating are pushing your fat intake sky high, that you eat fewer vegetables than you thought, or that you snack out of boredom.
Some people find that keeping a food diary is more than a fact-finding mission.
It can actually help you to change your diet. Writing foods down forces you to think about what you are eating. You might find that this alone causes you to make better food choices. Your dietitian will most likely start by taking a brief medical history and will ask you to articulate the dietary problem. Personally, I'm categorically opposed to weighing and measuring but would rather measure success in terms of diet and exercise behavior changes.