It
appears to be less true now than in days past, but humans are -first and foremost- storytellers. we like to tell stories, we like to listen to stories, and we -often- better remember lessons imparted in the form of a story.
Moreover, if you're trying to alter someone's opinion about the truth of a matter, you almost always need bolster your core argument with supplemental material. You often don't know much about your listener's background, you probably don't know what motivates him, and he may very well be opposed to your core argument for entirely unreasonable reasons. It's often best to surround your core argument with a large, somewhat varied structure, so that many folks will find some way to latch on to it.