In "Ode to the West Wind," why does Shelley call the West Wind "destroyer" and "preserver"?

In an apostrophe to the West Wind
(apostrophe: addressing someone who is not present or is dead, or
something that is not human as though present and living), Shelly is following an
established poetic convention of seeking the guidance, inspiration and attention of the
muse of the poem, in this case, the muse is the" wild West Wind." Whereas early poets,
through to at least the Elizabethan period, believed their divine inspiration for poetry
came from God and imparted spiritual truth that humans craved to know, Romantic poets
believed their inspiration for poetry came from nature and imparted truth about nature,
human nature, and philosophical musings.

Shelly describes
the West Wind as something that can be seen only through its force and power, like
driving leaves before the wind or blowing seeds to a cold grave. Shelly then turns the
topic of the first stanza and speaks of the West Wind's cousin, the Spring wind, that
will blow and summons to life the sleeping seeds that will raise "sweet buds" to the
air. It is in this context that Shelly calls the West Wind a Wild Spirit that is both a
"Destroyer" and a "Preserver": The West Wind destroys the peaceful landscape of autumn
by driving life (e.g., seeds, leaves) before it to cold graves, but it is a preserver
because it buries, or plants, the seeds of next year's life to be awakened by the Spring
wind that blows.