It moves because it can do none other, the speed of primary motion is nature's natural state of being. The primary particles provide structure for atoms which, in turn, provide the structure for all higher development. What can develop, within a particular environment, will develop. When all the conditions are met, development occurs. When an environment will no longer support a particular development, it degenerates, its parts and pieces becoming part of something else.
Development occurs upon some existing development, generally adding to the complexity of the entity.
Existence is a continuum of endless change, but of absolute fundamental principles and natural laws. Development and degeneration are continuing, ongoing processes. What can and cannot exist is set by the nature of the primary particles and the limited ways in which they articulate. Ultimate control is by these relatively simple mechanisms. There is no purpose in nature, as such, other than to do what it is and be what it can be.