im crying at how happy joker looks to see phyllis again
Nice theory! I have one thing to add to it: in the Betatraz ep we see that this purple quintessence is able to transform those who get in touch with it. The prison guard (whose name I forgot) gains size and power and his eyes turn purple. So I believe that the Galra use the raw quintessence to boost their bodies. They do harvest this, but we don't know yet WHY they do it.