Logic is important because when you're seeking to persuade, you want to make sure your argument makes sense. Being irrational is the worst trait to have when you're seeking to persuade.
Emotion is important because that's one way to persuade your readers. If you appeal to their emotions about a certain topic, it will be easier to persuade them.
Credibility is important because you don't want the readers to think you're clueless. If they do, you won't be able to persuade them.