It's an exercise in mind expansion...working dark to light. When working from light to dark, one often starts out sketching lines, then filling in shadows, mid tones, and finally layers of light. Starting with a dark background is definitely a method to stretch your creative abilities to be able to envision the process in a new way. It's almost like learning to see it all again, only backwards.
Many artists will find over the years a comfortable fall back position, a method of working that allows one to detach the mind and fall into creating. In order to keep refining, learning, and diving deeper into your art, it's beneficial to be a little uncomfortable. Work differently, use a color you don't like, raid that shelf of unfinished pages, and struggle a little. You may end up tired and frustrated, or you may, at the end of it all, open up a new channel and breathe new life into your art.