Question

Why does the jellyfish use bioluminescence as a defense,instead of remaining unseen in the darkness?

Answer 1

They use it as a defense because if they are being hunted by a predator, they will use a certain light pattern to alert any larger predators to get whatever is hunting them.

Answer 2

Bioluminescence is a light, right?

Lights on any animal, like a firefly or, in your case, a jellyfish, shows predators to back off and shows you that it's either poisonous or will hurt you. If it were to stay unseen in the darkness, predators would still not understand that it were poisonous or would hurt you

   ↑↑↑  Hope this helps! :D