I do it to pay the rent. But also: I've always had a need to express myself, whether through text or film or music. Since my mid-twenties I've tried to become more practical and realistic in what I decide to pursue, which sounds horrible on one level but is a must if I want to make a living out of it. This means, basically, to ditch vanity projects that have little or no chance of making it to an editor/publisher/producer, and focus on stuff I know has potential outside my cadre of friends. I'm also desperately trying to find my ideal kind of writing and concentrate on that; I am in awe of people who have found their calling, and can focus on that specific form of writing, such as plays, or movie scripts, or novels. I want to do it all at once, and I fear it's hurting my overall output.
Ironically, this post took me forever to write, and I'm not sure it even makes sense. I rarely try to explain/rationalize/defend my writing to anyone but me.