This technique of Steinbeck's is certainly not new to literature, but it is infrequently employed with modern American literature. Charles Dickens uses it in his A Tale of Two Cities in Chapter 3 of Book the First in which he reflects that "every human creature is constituted to be that profound secret and mystery to every other." This reflective chapter as well as others such as "The Wine Shop" act as descriptors for the setting and for characters, preparing the reader for significant themes.
In much the same way, Steinbeck prepares his readers for themes. In Chapter Three, for instance, Steinbeck describes a turtle whose tenacity is so strong that it plods along until it gets to its desired destination. Likewise, Tom Joad plods through the dust until he reaches home. The turtle, then, is symbolic of the people who plod onward until they procure jobs.
Consider one particular chapter that does not seem to relate and see how it actually does relate to the Joad family and the plot of the novel. For instance, look at Chapter 7 which deals with the selling of used cars to those, like the Joads, who need them desperately to go to California in hopes of making a new life after being driven from their homes. The chapter describes one used car lot, and it is written from the point of view of one faceless owner of one used car lot. Throughout the chapter, it becomes clear that this man lies and cheats to make as much money as possible from the misfortune of the migrants, whom he holds in complete contempt. He sells them cars he knows will prove to be unsafe and unreliable on their journey. He pays them little for their belongings, knowing they will have to accept whatever he offers. He forces them into signing contracts to pay more, when they have already paid too much. In his greed, he only wishes he had more cheap, defective cars to foist upon these desperate people at outlandish prices. Chapter 7 makes it clear what the Joads, and others, are facing as they attempt to flee the Midwest.
In later chapters, the car Al has picked out for the Joad's journey receives a lot of attention in the plot because it is vital to their reaching California without running out of money. In Chapter 13, Al drives "his face purposeful, his whole body listening to the car, his restless eyes jumping from the road to the instrument panel." Al becomes "one with the engine, every nerve listening for weaknesses, for the thumps or squeals, hums and chattering that indicate a change that may cause a breakdown."
Because we have read Chapter 7, we know what Al had been up against in choosing a car for the family, and the suspense is heightened. This car may very well break down. Al feels a great sense of responsibility, knowing he had to beat the vicious used car racket to find a car that would get them across the plains and through the mountains and the desert to California.
Look at it this way--there are two stories here. One is the story of the Joads, their trials and tribulations, the people they meet, the dreams they have. We learn everything about them as we follow them from Oklahoma to California. Look at the intercalary chapters as a second story--the story of all the others who were living the same stories, experiencing the same pains and joys and sorrows. The second story magnifies the first. Now it's not just one story but hundreds and thousands of stories. It was real and it was bigger than this one family.
Excellent points. I'd like to add the idea that the intercalary chapters serve as a magnifier to the Joads' story. A chapter on the Joads traveling the interstate is followed by a chapter on hundreds and hundreds of other cars doing the same thing every day. A chapter on the Joads running into trouble is followed by a chapter outlining all the trouble others are experiencing. The Joads buy one car, the next chapter has lots of people buying lots of cars. In other words, this is both one family's story and a whole lot of people's stories of being displaced and heading west to a better life--and what happens when they get there.
I would add that Steinbeck intentionally wanted the Joads to be representative of the crisis of the Dust Bowl. While we develop great empathy for the Joads, Steinbeck deliberately interrupts the narrative to pull us away from their individual crisis and refocus on the larger American tragedy, as mrerick points out.
Steinbeck uses an alternating chapter format to relate the events of the novel to the actual historical events of the US at that time. The historical chapters are called intercalary chapters.
It's an effective method because it forces the reader to see every aspect of this novel. It's a strong piece of historical fiction, but it's also a plain good work of fiction. The use of intercalary chapters allows the reader to see that what happened to the Joads was not only real, it was happening to many, many families across the nation. Without the historical chapters, we would be able to read the story of the Joad family, feel all of the emtions we're supposed to feel, then put it down and label is as good or bad like we do any other novel. When Steinbeck juxtaposes his plot to the historical atmosphere and attitude of the time, we, as readers, are forced to look deeper into the plot and recognize the struggles of the nation.
Basically, every other chapter of this novel is an example. We have an intercalary chapter that tells us about farmers getting forced off their land, then we see the empty Joad farm; we read about used car dealerships, then the Joads buy a car, and so on and so on.
I would reword your question - the intercalary chapters don't include the Joads, but I think they certainly deal with them. Those intercalary chapters provide the reader with a broad historical perspective of the US during the Great Depression. We, the readers, see an overview of the plight of the Okies and America, then a plot chapter relates that overview specifically to the Joads. The combination of the two types of chapters serves to show the readers that these historical problems happened to real people. If it were just the historical chapters, we would view it simply as "history". If it were just the plot chapters, we would view it simply as a story about fictional people. By intertwining the two, we can create a better mental image of the bad circumstances that actually happened to real families.