A journal is meant collect your ideas and observations on any number of things and put the happenings of each day into writing. In this way, you are able to better remember what you did, what you thought, and what was happening when you were younger.
Writing, like anything, improves with practice. When you journal every day, you're practicing the art of writing. And if you use a journal to express your thoughts and ideas, it'll help improve your overall communication skills.