It is essentially a whole other mixer without EQ or aux sends (maybe some EQ and Aux are assignable to the Mix B channels. That is usually used so you can have all your tape returns running into the Tape inputs and you can mix them seperately in the Mix B. That frees up your regular channels for the live tracks. You can also use it for extra inputs during mixdown that don't need EQ or much fader adjustment. Good for effects returns and playback of 2tracks.
It's also nice if you are in the middle of a mix and need to monitor a different project but don't want to screw up your fader and EQ settings on the main mix.
