I find you can hear phrasing and technique that you would not be able to hear without the isolated track. Even the players tone and how it is incorporated into the mix, you can clearly hear this as well as any possible effect that the player may have used.
Once you learn the track, through tab or sheet, (it toggles) which is provided in the cost. You can then interface with your device, strip the isolated track with a mute, and now you can record your part in its place. Then you can export your recording (mp3) to use however you want or erase it and start over again.
They offer frequent discounts and some tracks start at .99 going up to 5.99. Considering you get tab and sheet, this is not an exorbitant charge... especially if you use a discount code for some of the more expensive offerings.
* I don't work for Jammit, but I've met the people who have dev'd the app at Namm. Check out their videos.