If I feed the KPA a stereo mixed backing track, I will only get the right channel.
For example. If I were to play Lamb of God's Laid to Rest from my computer, I would miss out on half of the intro as the guitar is only in the right channel. That's an extreme but there is plenty of music out there with key elements only being heard through one channel. Pretty much everything the Beatles made in Stereo is an example that comes to mind as well.