Writing is like talking to an old friend, except that you don't have to worry about being interrupted by anyone or being told on the spot that your feelings aren't worth their time(that comes later during critique sessions with beta readers). Yes, I'm sure there are some gruff types out there who will deny this, but the rest of us, in our most honest moments, will cop to using our writing as a way to express that which we normally have trouble sharing.
As I've mentioned before, my daughter had some issues when she was born. This was a very difficult period in mine and my wife's lives, and sorting it out was hard. It took me a couple of years to understand what I was feeling, so I used writing as a way to do that. The premise of Salvation Day was born from that troubling period, so in addition to making the story much more in depth, I was able to use it as a way to sift through my emotions from that time and put them into a framework from which I could better understand them.
I've used writing from time to time as my own personal psyschiatrist. I've deleted some of what I've written as soon as it got on paper. Other stuff I've saved and re-molded into the background of a story. Of course the raw emotions will mean little to people who aren't me, and that makes the tale all that more compelling to write. I can give readers something they'll enjoy, but these things hold just a little more personal meaning to me. It's a way to make the story wholly my own while not hiding it from people.
Now, it's not all sunshine and buttercups. A couple of times when I've completely let loose, the results have frightened me. Some of the times when I've been at my lowest have been coupled with my darkest writing. These are things that will likely never see the light of day, and I'm even a little scared to go back and look at them. They remind me of times when I wasn't totally right in the head(as if I ever truly am) and I thought I could go no lower. However, they're a necessary part of my psyche. I could pretend those parts never existed, but I know that would mean they'd just resurface at the most inappropriate time, so it doesn't hurt to acknowledge the strange and dark part of us that we like to pretend doesn't exist.
(Imagine this person staring back at you from the mirror)
As I write, I'm coming to realize that a lot of people may look at this post and wonder whether I should already be fitted for a straitjacket, but those folks probably aren't writers. I'll bet that any person who is even moderately creative understands the value of writing as therapy, and even if they'll hide what comes out of their insanity, they'll acknowledge its benefits when they're alone. We know that while what lies underneath that facade we present to the world isn't always pretty, it makes us better writers and allows us to craft those crazy stories that people claim to adore.