1. Store all your best ideas in your journal. So often a great idea hits us and we write it on a scrap of paper. If you get into the habit of writing the idea and a bit of description of it in your journal, you'll always know where you can find that next great plot or article idea.
2. I'm sure this happens to you. More often than not, my head spins with ideas and all the things I want to accomplish. Use your journal to empty your head so you can make room for creativity.
3. If you are not sure of your writing, the journal is the perfect place to try it out. Maybe you've heard of a new writing technique, but you don't want to try it out on your paying audience. Try it on your journal. Write, write, write, and then leave it alone. Come back a few days later to see how it sounds to you.
4. Use your journal to re-write. Often a scene in a book doesn't work. A journal is the perfect spot to write it a different way or five different ways without disturbing your book. When you get it just right, insert it.
It's your choice whether you use a paper journal or a put your journal on your computer. The form is yours. The point is to have a place to practice your writing with the judging eyes of the world.