One of my more peculiar beliefs is that all game masters who possess the time and interest should also be writers. This is based entirely on my own idiosyncratic personal experience, and may very well not be universally applicable. But it's kept me out of trouble, and done a fair amount of good for my players, so I feel confident in recommending the practice.
It doesn't even have to be good writing. It may help if it's not. I've written two and a half novels, a play, and a novella, all spectacularly bad. That hasn't stopped me from getting a good deal of benefit out of them, both the practice of writing and the artifacts themselves.
In the first place, it encourages a creative habit. Not particularly necessary for an experienced game master, but it does a novice good. My first campaign was loosely based on the first (and most dramatically terrible) novella I wrote, and having that grounding of ideas on which to base a campaign made that early campaign much more successful than its inspiration. And my habits of thinking about stories and places and ideas for a long time, being used to doing "work," alone, as a source of entertainment, and writing down ideas as they came to me or as I stole them all started with writing, and all were handy as a game master.
It's not just being a former writer that's a benefit. Knowing that if an ideas proves to be unworkable for a game, I can just repurpose it for a story or something, keeps me focused on the table, and on ideas that make sense there. Likewise, there are certain things I'm uniquely terrible at writing about, but still call to me -- things involving guns and explosions, mostly. So action and adventure go in the gaming bin, and anything philosophical or sociological, (I read too much Heinlein as a kid, but I'm trying to kick the habit) focused on particular moments of emotion or decision, or that comes with anything resembling a plot goes in the writing bin. It also keeps me from getting too wrapped up in theme, though I'm starting to come around to the idea that there is a place for such things in a game. A decorative place, sure, but a place.
Most importantly, writing makes me appreciate just how easy running a game is. Running a game comes with a source of ready made ideas and information in the rules and whatever setting is implied or described by them, frequent but small deadlines, and a gang of happy, crazy people with ideas of their own and responses to mine. I have much more control over the finished product when I write, but that also means I have a lot more work to do. Writing is sometimes frustrating, often time consuming, and often hard -- and game mastering is, too. But compared to writing, it's a dream.