It's also a way of evoking an emotional reaction. Many Christians today would agree with the JWs that the image of fire with reference to hell is not literal (though most Christians who believe in hell would also assert that hell is eternal and conscious, not simply annihilation as the JWs teach). Hell is eternal separation from God and the eternal wrath of God - whatever form that might take. Fire isn't the only image used in the Bible - hell is also described as "outer darkness," which seems incompatible with the image of fire, if taken literally.
Emphasizing the "fiery" aspect of hell raises an image in the mind of the householder that evokes primal fears, which they may have been taught as a child or which are part of their cultural heritage (or even of pop-culture), but that may not be what their current churches actually teach. It's a form of straw man argumentation, since they are setting themselves up to argue against the most extreme forms of the doctrine, while, in effect, claiming victory against all forms when the householder agrees with them.