A rare lecture by Jony Ive in London back in the late 1990s where he explained that, when you were trying to sleep, the old sleep LEDs of laptops would blink on and off harshly, lighting up your entire bedroom each time which made it harder for some people to get to sleep and irritated people.
They therefore set out to create a more relaxing light which was not so aggressive and seemed more anthropomorphic.
As simple as this may sound, it meant going to the expense of creating a new controller chip which could drive the LED and change its brightness when the main CPU was shut down, all without harming battery life.
Most previous sleep LEDs were just driven directly from the system chipset and could only switch on or off.
Apple carried out research into breathing rates during sleep and used that figure to derive a model for how the light should behave to create the most relaxing atmosphere and make the product seem more human than robot.
IIRC it was introduced with the first plastic iBook G3 and has been with us ever since.