Intonation is used in basic grammar to indicate meaning. For instance, the sentence "You're here" means something quite different than the question "You're here?" The only difference orally is the pitch used when speaking the words. Chanting the sentences requires students to say them in a sing-song manner that helps the subtle intonation difference stick in their minds.
Pitch largely changes based on the context of what you're saying. By using a chant to teach intonation, you can teach students the differences between pitch and context in a way that will stick in their memory. Understanding intonation increases not only vocal competence, but listening comprehension, too.
Chanting can help students learn how to show emotion through the way they pronounce their words. An upbeat, lively chant can teach them the proper intonation --- for example, when singing "Happy Birthday" to a loved one. A slow, sad chant can teach them the proper intonation to be used when someone is sick or unhappy. A simple sing-song method can help the lesson become clearer to someone who is just learning a concept.
Have students pick a line from a text. It can be any line, as long as they can read it. Then, have them practice chanting the line, saying it with a different intonation each time. Repeat the exercise and each time discuss what intonation was used, then repeat that same intonation in the chant so that the students will retain the information.