95% of this book, you could say that Main Street features a limited third-person perspective, given that it seems to follow only the thoughts and actions of one character—Carol Kennicott. Yet there are a handful of places where the narrator leaves Carol and follows another character.
In chapter four, for example, we get this: "The train which brought Carol to Gopher Prairie also brought Miss Bea Sorenson" (4.3.1)—and then we find out a lot about Bea. Lewis likely does this to show us that it's totally possible for someone to have a completely different first impression of Gopher Prairie from the one Carol has. Instead of hating the place, Bea Sorenson thinks it's the greatest town ever, with tons of fun stuff to do. This puts Carol's problems into a different perspective.
The narrator's most significant break with Carol comes in chapter 21, which Lewis devotes to the backstory and secret pain of Vida Sherwin, who had a romantic fling with Will Kennicott just before Will married Carol. Again, Lewis probably does this to remind us that Carol is not the only complicated person in Gopher Prairie, even though she might think so. Everyone has their secret pains and disappointments (like Vida), but they just don't go around broadcasting them all day.
Lewis does the same thing again when he shows us the inner mind of Will Kennicott, who constantly feels belittled and humiliated by Carol's criticism. Now, it's clear from the narrative focus that Carol should be our main point of sympathy, but Lewis also wants to remind us that the people around her have feelings, too, even if she thinks of them as a bunch of self-satisfied fools.