For clients whose goals include appropriate weight loss, or for those with eating disorders, I think a food journal is essential for both you and your client to clearly evaluate dietary habits.
Keep in mind that food journals don’t always accuately reflect true intake patterns. One important point is to recommend to your clients that they record intake, both food and fluids, immediately after ingestion. Don’t wait until the end of the day. We tend to forget, not necessarily intentionally, a cookie here or a soda there.