Steinbeck advocated for the little guy. Almost all of his novels focus on the lives of people at the bottom. Migrant workers are and were some of the most disadvantaged people in the country, especially during the Great Depression. Many people during the Great Depression moved West in hope of finding work and employment. Many became migrant workers.
Steinbeck started his career as a journalist. When you read his novels, you can still find the journalist there - recording the details of the world and the struggles of the common man. He actually lived and worked as a migrant worker for a time (even traveling on trains like a hobo). The setting is what he experienced.
On a literary level, the farm really is a microcosm of the country. You are valued for what you can do; you are kept only so long as you can do it. Candy, for instance, is terrified that he will not be able to work and will be kicked off the farm. George dreams of having a place where he is in charge of who stays and who goes, a pace where they all can be safe from the utilitarian ideas of the farm/country.
The setting of 'Of Mice and Men' is significant to the author's intention in two key ways.
The setting of a text refers to where and when the action happens. Both the where and the when are significant to the text as a whole. By setting the novel in California in the Depression era (early to mid 1930's), Steinbeck is using the backdrop of a familiar setting to himself as a writer. Steinbeck was born in Salinas, close to Soledad, where the key events happen in the novel. This gives the setting realism, and the descriptions used are those known and seen by the writer.
Steinbeck was keen to reveal the reality of living through the Great Depression, and the effect that the economic down turn had on ordinary people. He spent time travelling as a migrant worker as George and Lennie did, and his characters were based on people he met during this time. Writing the novel in a real place and during the time which he himself lived makes the novel more than just a narrative, it also has an historical significance in its descriptions of the lives people led.