What is the function of the inner chapters in grapes of wrath?

These are there for some preaching.  After showing us
something, Steinbeck tells us about it.  It's part of a difficulty many novelists have
... getting information into a novel.  There are a couple other famous examples of
this.  In Brave New World, Huxley needs to tell us about how the people live and why
they live that way, so he introduces the book with the story of the hatchery.  It
doesn't have much to do with the story, but we need the background information to help
understand the rest of the book.  The same is true in 1984.  Orwell wants us to
understand "The Theory of Oligarchical Collectivism" and they only way he can get it in
the book is by having Winston read it in bed; that scene also functions to show the
difference between Winston and Julia's "war" with the government.  The "inner chapters"
do the same thing ... we get information/discussion about topics clearly related to
what's happening in the novel; it adds breath to the novel that might not be that
obvious otherwise.