"the sun spins mindlessly overhead, the shadows lengthen and shorten as if by plan" is an example of consonance. why does the author use this consonance?

Consonance is the agreement or compatibility between opinions or actions. I think again the very mechanical processes of nature are described here. This is the way Grendel sees the world rather than some deity lighting and darkening the sky.  Nature works as a mechanical entity that is compatible with its different processes.