Why do so many creatures in the pitch-black deep sea make their own light?

Sink far enough into the ocean and the sunlight runs out completely, leaving water as black as ink. You might expect it to be empty and dark. Instead it sparkles with living lights: glowing dots, flashing streaks and lit-up baits, all made by the animals themselves. Why would so many deep-sea creatures go to all that trouble to make their own light?