What purpose is the green lead inside notebook?

What purpose is the green lead inside notebook?

If you are chitchat about the green LED on the motherboard, it tell you the power status of your motherboard. Green meaning that it's ok. You manifestly don't want to turn on your PC if it's red as you might end up frying your components.
It shows that the computer is on.
so you know it's working... hehehe.. not the TRUE answer:

It may be for motherboard diagnosis (depending on the brand/type) When failing, some motherboards throw the error code (cause of failure) blinking that led (like some cars that blink the check engine frothy )