The main thing with renderlayers is that you can split up your render as it were.
Because most of the time the one or ones responsible for the look of a film/short you name it, want to do some colorgrading(adjusting colors so they look nice together).
You can also easily use another background for the render or adjust the shape/size of the background, to achieve better composition.
And there are probably a couple more reasons why you should render out in layers, but i think this is the main thing
So there you have it.

(Crappy school then by the way-if the teacher doesn't know stuff like this-)