Why do freshwater aquarium lights exist?

Lighting honestly really exists for two main reasons;

1) It allows us to look inside the water and see the fish. This also helps the fish regulate their day/night cycle and hunt, find food, and avoid other fish in the tank that may be larger than them through sight.

2) If you have any corals (for saltwater tanks) or plants, this will help them grow as without any light there aren't too many corals/plants that can thrive within the water column (at least, without having a pressurized tank for deeper depths).