One of the biggest benefits is the development of power. Muscular power is the ability to generate force quickly, and comes from running through mud and long grass, jumping puddles and obstacles, and maintaining pace up hills - all at race pace. If you've never worked on your power, you'll certainly notice the difference when the triathlon season starts again when you find yourself sprinting to transition or putting in a burst on the bike or run.
Cross-country is also good for core strength. Rough and uneven ground forces you to keep your balance as you run (although you may not even be aware you're doing it), and that involves using your body's core. A strong core will pay dividends next summer in everything from transitions (fiddly shoe-changing is much easier if you can keep your balance while standing on one leg) to your sprint for the line; your arms and legs work most efficiently when your body is upright and strong, not sagging.
Another benefit to be gained from the country comes from softer running surfaces, which mean you'll be far less likely to pick up the sort of injuries that can result from a winter pounding the asphalt. Of course, you can twist your ankle more easily when trail running, but on the other hand, uneven terrain helps develop foot and ankle strength.
Last but not least, your stamina will improve as you work to take each cross-country obstacle in your stride, no matter how thick, deep or fast they come.