The purpose of providing an optional set of audio layers in each track is to allow the construction of assembled pieces of audio that can have their playback relationships changed easily.
Audio layers behave a bit like opaque graphics layers in visual editing applications.
There are four available layers provided in each track - and they contribute to the main track according to the following simple rule. Any audio in a higher layers that is "visible" from the top of the stack will be heard. No audio beneath audio in another layer will add to the output.
One simple two-layer example is that a small section of audio in a layer on top of a longer section in a layer beneath the top layer, would act like a "moveable drop-in" and "cover up" whatever audio was below it. This would allowing a word or phrase to be replaced by the drop in, but not destructively.