how does the passage function? What purpose does steinbeck fulfill by beginning the novel in this way?

Which passage are you referring to? 

first paragraph of the novel 

John Steinbeck’s enduring popularity is largely the result of his ability to weave a complicated fictional reality from simple elements – simple language, simple characters, simple techniques. One of the techniques he uses consistently is the juxtaposition of the human and the natural worlds. He uses the first paragraph too set the simplicity of nature with the simplicity of the two men, Lennie in particular. The unforgiving simplicity of nature complements the narrative of George and Lennie's experiences through the book.