Why should you vary your running terrains? First it gives diversity to your running, but it also prevents injuries, improve stamina and performance. It can help build those little leg muscles which play important role for technique and balance.
Track is good terrain for interval training. Even if you are long distance runner, now speaking of marathon distance, you should do interval training. On the track you can easily set the distance, without looking continually to your watch. Track is also flat, which also helps making the good times.
Grass strengthening muscles, as it much softer than road, you need to work hard to move faster. Grass also prevent injuries, as roads are harsh for the legs. On the grass the impact is not as great than road.
Trails have the same benefits as grass, but they are even harder for legs, as there are more hills, stumps, rocks, etc. Trails are good training for the small balance and stability muscles, as each step is different. Even though trails have good strength and stability training, there is also a high risk of injury. So watch your feet and avoid slipping over.
If you are training, for road race, you have to get used to road. Road can feel tough and painful, if you are not used to it. Road don’t bend, which creates high impact your joints, muscles, tendons and bones. But roads are good for speed training and almost everything, you just have to remember give your leg break sometimes and run softer surface.
Dirty trails are often quite hard, but as hard as road, which makes it excellent surface. You can still have almost the same sense under your feet than road and more stable than trail. Even though dirty trails can be uneven some times.
Treadmills are soft and stable, you can adjust speed and incline easily. There are some disadvantages, for example, some treadmills are not really designed for running, when the belt might be quite short. Which makes runner go too close to the front, without leaving enough space for arms. Also many runners keep hand on the trails, leading wrong technique and even injuries.
Sand has quite same benefits as grass, but it’s often even more softer, which is not might good if you have any previous ankle or knee injuries. Also sand running often means beach running, and beaches are often diagonal. So running along the coastline, make your other leg is above the other, which why you have to run both directions.