I've got a bunch of the Fab Filter plugins and love them.   I notice Volcano is now on Offer -- but I can't figure out why I would need it - I have the Pro Q2 -- what can Volcano do for me that the EQ can't do?

The filters have more characteristics than those in Pro-Q, also there's the FF modulation wizardry area which Pro-Q doesn't have.  The filters have more flexible routing too.  Try a demo?

Volcano has full modulation capabilities, Pro-Q has none. You can drive Volcano from an envelope follower, an envelope generator or a sophisticated tempo-synced LFO. It allows you to radically alter the tone of pretty much anything, or add a rhythmic component to it.
 
If your genre is orchestral film scores, then Volcano might not be particularly useful to you. But if you're doing EDM, electronica or experimental music and you like to get technical, it could be an excellent creative tool.

ULTRABRA
but I can't figure out why I would need it - I have the Pro Q2 -- what can Volcano do for me that the EQ can't do?

That's a question I asked myself at one point.  I got Volcano in a bundle along with two other things I actually wanted, but couldn't figure out what it was good for.  I ended up writing this as a result of my finding answers to that question:
 
http://soundbytesmag.net/effective-effect-filtering/

Here's the video. They demonstrate using the LFO to drive the filters.
http://youtu.be/dRLUE1xfAQU