Why do we put "2 * Mathf.PI" ?

I bumped into this statement in “Light.intensity” 's script …I try it out and it worked just fine even if we drop " 2* Mathf.Pi" out, so why do we use it for?

var phi : float = Time.time / duration * 2 * Mathf.PI;
I know it has to do with cycle, or full cycle… PLEASE

That gives you a value in radians which is commonly use in angle equation.

You have a full circle with 2*PI that you cut in quarter of duration. Then you multiply by the elapsed time and that give you a value in radians of the elapsed time.

Fact is yes, you do not absolutely need it. But then you have a simple value no more in radians.