Which light? The one above your tank gives light to your plants, if you have a planted tank, lets you see the fish in a non-planted tank. If you don't have plants, fish do fine with just room light.
The one on your heater, if you have a heater, or one with a light, lets you know if the heater is on or off.
The light in the room is there so you don't bash the snot out of your toes going to the bathroom in the middle of the night.
Tolak