Question:Why was light used for communication historically?
Because light travels very quickly over large distances.
You cannot shout from one mountain to the next but you could see a signal made from light. Even if it was just a warning fire.
Light could also be used to relay message over long distances by having a series of people along the way to pass on the message they saw using more light.
Later on Morse code was used to flash messages from one place to another.