In the beginning, I pretty much hated this. It felt like one more thing I had to do each day. But after about two months, it became a useful part of my routine. Sometimes I vent. Sometimes I daydream on the page about my work-in-progress until I can barely wait to work on it. Sometimes I think of ideas that get transcribed onto my to do list. It’s like a mini-brain-dump on the page. I like using a cheap spiral-bound notebook (rather than a nice journal) so I don’t feel pressure to record anything "important."
(As an aside, some people use Bullet Journals which is more of a way to track tasks and goals than it is a free-form writing journal. I have yet to experiment with this, but you can learn more about it at Bullet Journal--Getting Started. )
These two articles link journaling to productivity: The #1 Productivity Tool You Aren't Using and Start this Simple Habit to Rocket Your Productivity: Journaling.) I do think it helps me have a certain positive mindset to start the day and increases my focus for the creative work ahead.