Moss is really neat, because it can tolerate the very harsh conditions that are common in the desert. They can survive being desiccated (almost completely dried out), extreme heat, and freezing. As soon as conditions improve, they perk right up! That means that when it rains, moss can quickly "wake up" and start exchanging nutrients with the soil and water. (You can see how quickly dry moss "wakes up" after I add water in the video below.) Other, larger plants are not able to respond to rain quite so quickly.