To present the argument cogently and artistically.
The canon of style concerns the choices rhetors make to form statements that will have calculated (surmised) effects on the audience. Style is most often thought of as making choices about the levels of language, i.e. grand, middle, and low. And style also concerns the choices one makes of tropes and schemes.
A concern with style is a concern with eloquence matched with kairos. But I would argue that style is more than the "dress of language" as eighteenth-century rhetorician Hugh Blair claimed. We may dress language in revealing or obscure garb. Thus, style may also be implicated in the socio-political intent of a message.
For example, let's examine a statement from John. F. Kennedy's inaugural address: "And so, my fellow Americans, ask not what your country can do for you; ask what you can do for your country."
We could begin a short critique of this statement by noting that it has a grand "sound" or style. That grandness is achieved with two schemes: anastrophe (inversion of normal word order) and antithesis (juxtaposition of contrasting ideas). I would guess that many Americans, even young people, can associate this statement with Kennedy. I have discovered in my classes, however, that students do not know the context. They often assume that it has something to do with Kennedy's call to national service and the creation of the Peace Corps.
Actually, the statement is the concluding remark following paragraph 24, in which Kennedy calls upon Americans to defend "freedom in its maximum hour of danger." This is a call to be vigilant during the early Cold War period.
Also, note that Kennedy's statement could be construed as a rather dubious assertion about the proper relationship between a citizen and a democratic government.
But, we remember the line (often out of context) and revere it (despite its political difficulties) precisely because it sounds so good. This is stuff of soundbites and headlines. Rhetoric scholar James A. Berlin said that "language is never innocent." Style is more than simply the dress of language.
At the top of my syllabi, I reproduce this lyric by Sting of the rock group The Police:
I suggest to my students that Sting has made an error (caused by the need to preserve the meter). The last two lines should read: 'Cause when their logic escapes you / Their eloquence ties you up and rapes you.