According to Wikipedia, homiletics is “the application of the general principles of rhetoric to the specific art of public preaching”. For those who have not studied rhetoric, its general principles are usually broken down as follows: ethos (the establishment of trust and credibility), pathos (appealing to the emotions) and logos (appealing to reason and logic). So then, homiletics has to do not so much with the content of the message, but rather with its composition and delivery. It is about taking a proposition and making it persuasive.
For Christian preachers, the starting point is the truth of God. Homiletics is about adorning it rather than veiling it, undermining it or otherwise sabotaging the attempt to communicate it.