Witing is essentially interior work, and many writers are interior personalities. Having a job forces you out of the world of your work, and into the one in which you get to observe people. Yes, you can imagine all this, but as a fiction writer, you can never observe enough the rhythms of how humans move through the world. A job demands that you structure your time much more carefully. You learn how to be resourceful, and that in turn provides a certain intensity of focus.