There exists consensus among writers regarding the importance of keeping a writer's journal, but none that I have read—Thoreau, Kafka, Emerson, Plath or the great wit Oscar Wilde—expresses what I find to be of greatest value to the task.
Frequently mentioned is the the worth of maintaining a repository of ideas for future work, the merit of flexing one's writing muscles, and working through self-doubt and writer's block. C.S. Lewis and Anaïs Nin both refer to journaling as a drug, which abstractly touches on what I find to be the greatest benefit of the task. Maintaining a writer's journal while actively working on a writing project, provides a dumping ground for the ego—a way to separate the self from the work, thereby avoiding the embarrassment of being caught, mid ego stroke, by one's own audience.
The pursuit of any art is an inherently egotistical endeavor. And, at its best, writing is an art form—whether a corporate communication or a poem. When writing for other than pure pleasure, recognizing when your ego makes an inappropriate appearance, can stop you in your tracks, disrupting your flow. A good way to counteract this is to pick up your writer's journal and throw your ego onto its pages, before turning back to the task at hand. Next time you find your writing straying into self-indulgent territory, give my method a whirl and let me know if it works for you too!