Personally, I think a change of tone is often used to engage listeners.
How many times have you sat listening to someone reading, either in class, a lecture or even in church, and all you really concentrate on is the drone of their voice?
Most pupils at my daughter's school, tend to read in the same sing-song voice and I am following the flow like a song instead of listening to what they are actually saying.
A similar thing happens when people repeat the same word often. One of the teachers (again at my daughter's school) tends to put 'you know' into almost every sentence, so instead of listening to what she is trying to get across, I am counting the number of times she says "you know".
Changing the tone of the voice makes people sit up and listen, if they are bored or feeling sleepy, due to the heat of the room perhaps, a change in tone will jog them slightly and they will again take note of what is being said.
It's a technique used by many public speakers. Here's a video of Barack Obama giving a speech - notice how many times he changes the tone of his voice?
