In the first canto of "Ode to the West Wind," Shelley presents the wind as a harbinger (messenger) of change. Shelley noted that the turbulent rising wind of such an autumn storm would "pour down the autumnal rains" (Norton Anthology of Romantic Poetry). The autumn is associated with death; to be followed by autumn's "azure sister of Spring," a time of new life.
The arrival of the wild West Wind announces the autumn. The wind is the "breath of Autumn's being" (1). The wind, unseen, drives the leaves "like ghosts from an enchanter fleeing" (3). The wind, "enchanter," is personified as a conjurer driving out the dead.
In the second stanza of the first canto, the multi-colored leaves are driven to "their dark wintry bed" (6). "Hectic" red refers to a fever that would occur to those suffering from tuberculosis. So, driving out the leaves symbolic of death and a cycle of nature but also a cleansing.
The seeds which lie like corpses will blossom in spring, completing the cycle of death and rebirth. The wind is both "Destroyer and Preserver" because it brings death but this is to be followed by new life.
The West Wind is described as "wild" in the first line. This indicates a natural, perhaps sudden, and dramatic change. In recurring themes of Romantic poetry, and certainly in Shelley's, this natural event can be analogous to a burst of poetic creativity or a revolution/improvement of humanity. Shelley indicates, more specifically in the later cantos, that this death/rebirth is a metaphor of social change. He supposes his poetry might undergo a forgotten time (winter), in the near or distant future, to be followed by a reemergence (spring):
Drive my dead thoughts over the universe
Like withered leaves to quicken a new birth! (63-64)