Why does Shelley invoke the power of the West Wind?

Shelley sees the West Wind as sublime, powerful as it blows the autumn leaves from the trees, and powerful as a force for change. Shelley invokes this power because he wishes the West Wind would blow the "leaves" or pages of his verse over the earth with the same power that it scatters the "leaves" from the trees. These leaves are dead on autumn trees, but the wind seems to blow new life into them as they are scattered. So Shelley wishes the wind would breathe new life into what he calls his "dead" words: 

Drive my dead thoughts over the universe

Like withered leaves to quicken a new birth!

Of course, he doesn't literally want his writing blown about by the wind. The wind functions as a metaphor for a force equally as powerful to spread his thoughts and ideas throughout the world. (He probably would have loved the Internet.) He also, himself, would like to be a force of change like the West Wind. "Be thou, Spirit fierce, my Spirit!" he writes. "Be thou me..."