The McClellan family, and especially Clarisse, serve as a foil for Guy, his family, and his coworkers. Since the novel is mainly written from Guy's point of view, the way Clarisse's family is described indicates what Guy finds unusual about them, and therefore what the norm is for "normal" citizens of this dystopia. For instance, in Part 1, the McClellans are described like this:
Laughter blew across the moon-colored lawn from the house of Clarisse and her father and mother and the uncle who smiled so quietly and so earnestly. Above all, their laughter was relaxed and hearty and not forced in any way, coming from the house that was so brightly lit this late at night while all the other houses were kept to themselves in darkness. Montag heard the voices talking, talking, talking, giving, talking, weaving, reweaving their hypnotic web.
Guy's shock at Clarisse's habits indicate to the reader how different this world is from ours. Note in this passage what is mentioned about Clarisse and her family (and therefore what Guy finds remarkable about them). Their laughter is "not forced," which suggests Guy hears lots of forced laughter in the families he's used to. Their house is still lit as they have an involved conversation with each other, an activity Guy finds odd. Their talking is a metaphorical "hypnotic web." The metaphor suggests that Guy wants to join this appealing world, but also finds it dangerous and fears getting trapped within it. Guy is interested in words specifically: eventually he'll seek out books, but right now, it is this passionate talking which interests and frightens him.