Who is telling the story, and what is the value of having alternative voices in the narration?
The value of having alternative voices is that Steinbeck can change the tone and mood of the point of view to suit his purpose, whether it's to include social and historical information or to dramatize what the Joads are feeling.
Through the different voices, he is able to better reflect the theme of the novel. The Joads represent the need of all men and women to find some kind of dignity in their lives. These chapters allow him to give the reader necessary background information of the Depression era so we can understand its effect not only on the Joads but also on all of the migrants like the Joads. When Steinbeck wants to focus on the Joads in particular, the point of view then becomes more restricted, allowing the reader to feel what the Joads feel.
This method also allows Steinbeck to include much of the symbolism and conflict in the novel. In Chapter three, the journey of the turtle crossing the highway is symbolic of the journey of the Joads and other migrants to California, demonstrating their will to survive. We also see the conflict between the migrants and the bank.
By changing the narrative between different points of view, Steinbeck is able to tell his story more dramatically. It provides a way to include the literary elements of any work of fiction so it's more enjoyable to the reader.
The Grapes of Wrath shifts dramatically between different points of view throughout. Events are described broadly in some chapters by the narrator by summarizing the experiences of a large number of people and providing historical analysis. Frequently, in the same chapters, the narrator assumes the voice of a typical individual, such as a displaced farmer or a crooked used-car salesman, expressing that person’s individual concerns. When the narrator assumes the voice of an anonymous individual, the words sometimes sound like what an actual person might say, but sometimes they form a highly poetic representation of the anonymous individual’s thoughts and soul. The chapters focusing on the Joad family are narrated primarily from an objective point of view, representing conversations and interactions without focusing on any particular character. Here, the characters’ actions are presented as an observer might witness them, without directly representing the characters’ thoughts and motivations. At certain points, however, the narrator shifts and presents the Joads from an omniscient point of view, explaining their psychologies, characters, and motivations in intimate detail.
Also keep in mind that this style of storytelling was slightly out of character for Steinbeck. Compare this novel to his other more famous ones, and you'll find much more historical context in GOW than any other novel. There are some critics that believe this method of switching between plot chapters and intercalary chapters takes away from Steinbeck's ability to accentuate characterization (one of his strongest assests as an author). Other critics believe that providing these historical scenes actually does the opposite: makes his GOW characters more real as the audience is able to connect them to previous knowledge or experience.