Journaling as therapy?

While I was driving to work this morning, I was listening to a weight-loss podcast that suggested everyone attempting to lose weight should keep a daily journal. In the journal, write about your day, what you ate, and why you ate it. This allows you to reflect on your food choices, see where you went wrong and what you did right, so you can do better tomorrow.
I found this to be an interesting thought. Does a journal help you? Do you write on paper, or use a blog? What are your goals for your journal? Are you simply enjoying writing? Or are you using it to preserve your thoughts? Or perhaps just using a journal as a daily log for reference?
It occurred to me that a journal can be much more than what I originally thought of one: a basic record of the day's events. It could be a tool for so many other goals: helping you focus on career advancement; as a school aide; as a weight loss aide, and so on.
The podcast also stated that it is extremely difficult to lie on paper. With all the junk I read in the papers, I wonder if this is true. For a personal journal, it might be. But this certainly is not true for our media today. Perhaps the media should keep a journal, with the goal of honest reporting?