Why does Shelley say that the west wind is at once "destroyer and preserver"?

The west wind is called a destroyer as it destroys the decaying leaves of the trees in autumn. It drives away the rotten and withered leaves to distant places. The poet suggests by this that the mighty force should destroy the rotten things of our society. Again it called preserver because it carries the seeds underground where they lie until the advent of spring. In spring the whole Nature puts on a new appearance of freshness and the hills and the plains are covered with beautiful flowers.