Symbolism, Imagery, Allegory
Is it just us, or is François really creepy? He keeps popping up in really important moments in the novel. Like, in Chapter 30, the constant meowing of François drives Laurent crazy. And in one of his weirder hallucinations, Laurent becomes obsessed with the idea that if the cat could talk, he would expose him as a murderer.
No, we're not kidding. The guy actually convinced himself that this cat, like Mme Raquin, knew about his crime, and would denounce him some day if he were to ever learn to speak.
But we think that the cat is a container for Zola's ideas about the differences—or lack thereof—between humans and other animals. Note that he states there is "some resemblance between the cat and the paralysed woman" (30.20).
What exactly is the resemblance between Madame and the cat? Well, for one thing, they're both talking animals, of a sort. See, the old woman should be able to speak, but she currently can't. And the dumb cat can speak—or at least Laurent is really afraid he could learn.
So Zola is using François to poke holes in the idea that language is what ultimately separates us higher-thinking humans from animals. To him, people are just animals who happen to talk.