I started keeping a journal when I was very young, filling the pages with pre-adolescent events in my life like I went to the beach. I had fun. I stopped when I began thinking of writing professionally and was spending so much time writing.
In the last couple of years, though, I began to journal again, and I've re-discovered the freedom of writing anything that comes to mind and not having to be concerned about revising and editing. I've re-discovered the freedom of knowing that no one other than myself is ever going to read these pages. I've found another benefit to journaling that has helped in my writing - writing out plot problems.
Often writers work alone and we don't have anyone to talk to about a problem with plot or character. I'm "talking" my problems out on the pages of my journal. I'm finding that writing the problem down and through helps me solve it.
I'm no longer talking out writing problems aloud to myself. Well, not as often : - )