I wrote for nearly six hours. When I stopped, the dark mood, as if by magic, had folded its cloak and gone away.- Zane Grey, born on this date in 1872
Zane Grey was the first "adult" writer I ever read. I was ten and I loved horses/cowboys. I think this quotation nails something and explains why so many depressives (with talent) find expressing themselves in words/paint/whatever keeps the demon at bay.
Not so much keeping "the demon at bay", rather allowing the "dark mood" full run so that the task can be completed, which was the problem to be resolved in the first place.