Question about import flash.events.MouseEvent;

[An Artist]

What is the purpose of a import flash.events.MouseEvent;?
I am learning Actionscript 3.0 the tutorial tells me to use import flash.events.MouseEvent;. However, when I take it out it still works.
Thanks for any help.

stop();
import flash.events.MouseEvent;

//--- G  Button Timeline change--\\
//--- R Button Property change--\\

R_btn.addEventListener(MouseEvent.CLICK,rClick);
R_btn.addEventListener(MouseEvent.MOUSE_OUT,rOut);
function rClick(event:MouseEvent):void{
	R_btn.alpha-=0.1;
}

function rOut(event:MouseEvent):void{
	R_btn.alpha =1
}