Joel Zartman says that we do, partly because we cannot trust ourselves. Read the whole thing at Unknowing.

Which is why you need rhetoric. You have to be conscious of it if you want to do it at all well, unless you naturally do it well. It requires consideration, and consideration beyond what you considering can come up with. And even if you naturally do it well, it is well to consider what is proper and improper not only in the aesthetic realm—which is important—but in the moral realm. If you don’t think about whether you ought to achieve things by one or another effect, if you don’t label certain approaches and understand clearly what you are doing, how will you confront your deceitful heart when you are proceeding by sensationalism, fearmongering, omitting crucial details and such unscrupulous means?