Life and all of its creatures are dynamic. We move, and our movements, even the subtlest of ones, express emotions. If we were to remove such things, even the tiniest smile, life would become dull and static. Animation effects can range from the simplest of things such as a flag waving to a dragon flying. Just look around you, inside and outside; something will be moving or will move from time to time. Even stones move, albeit with wind, something knocking them, or someone skipping them across water.
Unity has a complex animation system, also known as Mecanim, which requires time to get used to. It includes different components. Some of these are specifically for 3D animations; others can be used for both 2D and 3D. Usually, animating in 3D is harder than in 2D since it requires tweaking of many parameters, and therefore much more practice is needed to master it. If you are interested in learning more about 3D animation in Unity, I suggest that you to read a specific book about...