Well, obviously, if you commit to write a novel, you write nearly every day for weeks or months on end. That much practice can’t hurt! As Malcolm Gladwell writes in Outliers, it takes 10,000-hours of practice to gain mastery of a skill.* Writing a novel certainly burns through those hours.
It also keeps your creative pot in a constant ferment. The creative side of your mind is always working to come up with solutions to problems in your novel: ways to deepen characters, close plot holes, intensify action. And when you need to apply creativity to other endeavors, especially other writing projects, you’re already spewing ideas. I think this may be what rock musicians refer to as “road chops,” the idea that the best time to record an album is when you come back from a tour.
Finally, I think writing a novel forces you to write scenes and situations you wouldn’t normally. When you write a short story, the scope is so limited you can really choose the scenes you feel most interested in or comfortable writing. But in a novel, you’re always coming to places where you have to write beyond your comfort zone, simply because the plot is so involved and the characters so numerous. That, even more than the first two points, is what really stretches your writing skills.
* This may not be a rule Gladwell actually came up with him himself; apologies to whomever he cribbed it from if that’s the case.