The intercalary chapters appear throughout The Grapes of Wrath in order to help the reader understand the greater scope of the time period. For example, chapter 1 gives the setting of the novel by describing the Dust Bowl in detail.
While the chapters dealing with the Joads let us know more about the plot and characters, the intercalary chapters give us a reference outside just one family and show the greater societal influence of those same actions and events. Chapter 14 is a good example of this, showing the migration of the people of Oklahoma to California along Route 66.
In other circumstances, the intercalary chapters offer more symbolic significance of foreshadowing, as with the turtle in chapter 3.
When you come across these intercalary chapters, ask yourself the deeper meaning Steinbeck is conveying to the reader through the details included.
These chapters depict figure without names, for the most part, helping to demonstrate the idea that what is happening to the Joad family is happening to many other Americans. The plight of the farmers in the Dust Bowl is a generalized plight. The intercalary chapters also show that this plight is not attributable to nature primarily but can be attributed instead to corporate greed, banking policies and politics.
In defining the terms of the underlying conflict of the novel, these chapters also help to develop the novel's central theme of struggle against a faceless system (and/or struggle to maintain one's humanity in the midst of a dehumanizing system).
We can see this theme articulated in Chapter 5, where the mechanized tools of farming (tractors, etc.) are characterized as monsters and the men hired to run them are depicted as inhuman as well.
"The man sitting in the iron seat did not look like a man; gloved, goggled, rubber dust mask over nose and mouth, he was part of the monster, a robot on the seat."
In passages like these, the intercalary chapters help to articulate the idea that the Joad family and those like them are engaged in a struggle for and about humanity. They are not only attempting to survive and to find work, they are challenged to maintain a sense of humanity along the way (i.e. sympathy, polity, generosity, compassion, etc.).
While many of the interstitial chapters deal with corporate indifference and a mechanized threat to a human way of life, there are others that depict the positive values that the Joad family often also represents. Chapter 15 is an outstanding chapter, remarkable for its concise and touching demonstration of how sympathy does still exist in the world of Grapes of Wrath.
In this chapter a diner is featured and the terse owner/cook, Al, and a waitress, Mae, are approached by a family of people on their way out west. Al insists on giving the family a fifteen cent loaf of bread for ten cents, which is all the family can afford. Then Mae gives the children candy, lying about the price to make it cheaper for them.
Their generosity is immediately recognized by two truck drivers in the diner who leave large tips. When the truck drivers leave, Al goes over to a slot machine and plays it, saying that "Number three's ready to pay off." He plays the machine a few times until it pays off and goes back into the kitchen.
The actions of Mae and Al show that they are imperfect people but they are willing to help those in need. They do not treat all their customers as if they were each identical (as the powers-that-be in the novel so often do). In recognizing the individuality of the people they encounter, they reinforce the idea that it is possible to understand the plight of another. It is possible, also, to help people even while taking care of one's self (as Al does by manipulating the slot machines).
The intercalary chapters are all the odd ones, except 13 and including chapters 12 and 14.