Aristotle has written many different discussions in many different sections about one specific topic: Style. How you form it, how you personalize it, how you use it to enhance your argument, you name it- he’s covered it all. Most of the advice, however, had been about the format or flow of the style, and not the intonation, which is why Aristotle dedicated a section solely to the subject.
Usually, Aristotle centers his thoughts around the importance of using the right words, arranging them to best persuade your audience, and including writing enhancements such as metaphors, maxims, or examples. What he talks about in this section is how you speak these words. He tells us to sound dramatic when delivering a dramatic argument, sincere when talking about a subject of great importance, passionate when calling people to action, and caring when garnering sympathy from your audience. The tone of your voice can completely change the reaction of listeners, and it can be used to achieve the reaction you desire. It embodies the vibe of your argument and it plays a crucial part in oratorical arguments. Just as it can improve your argument, Aristotle warns us against the problems misuse of intonation can cause: imagine a speaker using a tone of caring and compassion when trying to rally an army- it would pretty much defeat the whole purpose. He says this not to discourage use of intonation, but to encourage thoughtful use of it.