The VC Mixer is pretty cool. It has some functionality that allows you to do do some stuff that you might otherwise need a separate module for. Here's an example:
I often want to have the filter cutoff be modulated by both the EG and the LFO, but you can only have one or the other on the VCF MOD SOURCE switch. Bummer.
But wait! Patch panel to the rescue - I'll just patch LFO TRI out to VCF CUTOFF that works! Except...damn, I don't want it full strength Bummer.
But wait again! VC MIXER to the rescue! I'll patch LFO TRI out to MIX 1 in and CV MIX out to VCF CUTOFF. Now I can use the VC MIX knob to blend in however much LFO I want on cutoff.
Yay Mom! You're so cool!
PS - Check out page 17 of the manual for a complete description of the mixer. There are other great ideas for its use in there me thinks.