In addition, the conversation also provides an element of foreshadowing, as their discussion of the Misfit soon turns into an actual encounter with him for the family.

The conversation with Red Sammy adds a few things to the plot.  First of all, he is the first to introduce the title of the story "A Good Man is Hard to Find" when he and the grandmother are discussing the state of the world.  This scene also shows the grandmother's initial idea of what a 'good man' is - she declares that Red Sammy is a 'good man' because he let someone charge gas even though the description of Red clearly proves he is not a 'good man'.  It shows that the grandmother's ideas are empty, ignorant, and vain.  Also, this conversation again brings up the Misfit, who everyone pegs as the 'bad' guy, but the grandmother declares he is 'good' before the end.  The author demonstrates throughout the story that all humans are equally in need of grace because we are all very flawed.  And finally, the conversation again reveals the prejudices that the grandmother has.  Pointing out prejudice is one of the author's themes in this work.