The way I process information and emotions is mostly verbal. If something significant happens, like the birth of my son, and I do not have anyone to discuss it with or take the time to write it down, I get very jumbled in my thoughts. It can effect my relationships, that's how important it is. Writing, for me, is not only an expressive outlet, but also a processing plant for sorting emotions and meaning. If you're anything like me - sensitive, highly emotional, often irrational, etc. - I highly recommend journaling at very least. I understand that not everyone is like me in that way so if I haven't convinced you yet, hold on.
Recently, someone commented that blogging wouldn't be possible because everything they went to say just needed to be deleted. In our instantaneous world where your words are plastered in front of everyone's eyes in a matter of seconds, being careful about what you say is vital. Still, there is something that happens when you sit down to write, attempt a thought, and then read it, only to decide it should be different or deleted altogether. For me, I regularly discover areas where my thoughts are unhealthy, incorrect or just down right sinful. When all I want to do is gripe about somebody or some situation in my writing, it becomes clear pretty quickly that I need to change. All I'm saying here is that writing and disciplining that writing for others eyes helps me to grow. It helps me be more honest with myself and see my thoughts and actions more as they are, rather than how I want them to be. And to be honest, I hope that things I write about are interesting at least some of the time, but I write for me. And I think that's important.