Why might it be helpful for some fish to be luminous?

it will be helpful for some fish to be luminous because if they life at the bottom of the ocean, no sun reachs there so then they will have to have their own light. they also might need to luminous because if another fish comes by to fight or harm the fish and its "family" it will use its light to scare the fish away and the light works the other way sometime it might being smaller fish of stuff closer to it so it will have eat it or something like that.