Certain items open up new areas of the game. Think of it as delving further into an unknown spec tree, or AA's even. That's truly what it is, alternate advancement. On top of that you learn game mechanics beyond what one normally would.
There are several items out there that allow you to do this. CoS for nec's is the easiest, vp staff, beads, manna robe, etc.
All of these items further progress your abilities to do something you couldn't previously do without them in the game and it's a real treat when they all come together like a symphony of badassery.
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