You can't change anything but the tempo and the overall key. ( haven't tried either yet ) The real benefit is two-fold.
1) Play along with real audio files so you can include vocal harmonies, actual solo instruments etc..... Great sound
2) Play DJ style on breaks. Load up stuff that you can't ( or won't )do live and play them on the breaks while you give the folks a break from your voice !

Think of it as an SMF sequencer that plays real audio files.