This lets you play multiple BGMs at once stacked on top of each other. It is another way to do dynamic music changes. In Soundtrack Manager, you can do dynamic music changes only by swapping between completely different tracks.
I wanted to put up a demo on YouTube to show how it would work, but the tracks are in need of a bit of refinement. What I'd intend to do with that demo is a feature I'm implementing into Jay's Journey: each playable character, when in the active party, adds an instrument to the "Ready to Rock" battle theme.
Let's say I was trying to do that with Soundtrack Manager alone. And let's say there are exactly 10 playable characters (Jay's Journey and Jay's Journey 2 combined have more than that.) This would require no fewer than:
* 10 different tracks for just one character present
* 45 different tracks for two characters present (10 * 9 / 2)
* 120 different tracks for three characters present (10 * 9 * 8 / (3 * 2))
* 210 different tracks for four characters present (10 * 9 * 8 * 7 / (4 * 3 * 2))
So 385 different versions of the battle theme - and that number only goes up quickly with each additional character! Clearly not worthwhile.
How many tracks would that take with Soundtrack Layering? 11. One base track and one track for each playable character. Adding another character adds one more track.