Few cottage industries are as hyperactive as the iPod case business. And when you combine one with the irresistible urge for people to make public fools of themselves with their music, you may have a winner on your hands.
The "Blue Beatz" case may have found this unfortunate sweet spot with a protective shell that doubles as an amateur DJ device. As Chip Chick says, you can mix multiple tunes to play simultaneously for some undefined reason.
We speculate that this "feature" was really a mistake that the manufacturer is trying to turn into a marketing opportunity. It certainly wouldn't be the first time that's happened.