Marketing mostly. Good speaker designs don't "need" it, but many speaker companies simply provide the terminals cuz everyone else does.....
Bi-wiring cables are one terminal at the amp end, but dual at the other.....the passive bi-amping thing uses separate terminals on the amp. More a waste of wire than anything, but whatever floats your boat!
I suggest you read the AH article I linked....it's not really theory, but explains how they function....