I'm of the opinion that every game master should write at least one novel.
Or make at least one movie. Or one comic book. Or video game. But a novel is one of the simplest, most solitary, and easiest to complete narrative forms around. (And it will be particularly easy to complete when NaNoWriMo rolls around.)
Trying another narrative form is a good way to pick up tricks applicable to running a game. Character development, world design, the generation of conflict; a novel improves all these skills.
But more broadly, knowing another narrative form is a good way, maybe the only way, to find out what roleplaying is really good at. What can I do with a campaign that I can't do with a novel?
Once I've answered that question, those are the things I should focus on. Because those are the things that players can only get from my campaign. Those are the things that will keep them coming back.