Happier joints and injury prevention: Training on softer surfaces reduces the pounding on your legs. Many elites run on dirt because soft trail miles are 'easier' on the body. With the amount of work they put in, the surface makes a world of difference.
More supple joints: Navigating twists, turns and uneven surfaces strengthens the smaller muscles, tendons and ligaments around the ankles and feet. This, if done gradually and smartly—don't go from no trail running to all trail running or your feet won't have time to get stronger gradually—will make your ankles and feet stronger and less prone to a sudden pull or sprain if you step in a pothole.
The mental factor: A change of scenery is always nice and, if you get going on a good trail, that long run can 'feel' shorter.
Sign up for your next trail race.