This can be really beneficial when transcribing, as it's possible to cut out half of the instruments and isolate more of what you want to hear. A little bit of fiddling with the EQ can make this even more effective. I've done this quite a lot in my own transcribing, but for some stupid reason it never occurred to me to do this in reverse.
The idea struck me the other day as I was transcribing "Cidade Vazia", and in playing around with it I figured out how to take this concept a step further in order to make play along tracks.
In this recording, the piano and bass are panned hard left, with almost no drums on that side. So I dropped the track into GarageBand and panned the channel hard left. I then copied the file to a new track and, using a built-in GarageBand plugin, flipped the signal so that piano and bass were in the right channel, and then panned that track hard right. What I was left with was a version of the song with the drums almost completely gone. I then dug into the EQ, bringing the bass up a bit amongst a few other things, which further pronounced the effect.
So, I now have the original Milton Banana recording of "Cidade Vazia" with almost no drums in it, essentially creating the best play along track you could ask for, and I share it with you today. Check out the transcription in the previous post and enjoy!