Confusion on the Tutorial, Menu with Slider

Well, please check this tutorial :

In the tutorial, we add this code ( in step number 7) :

button_1.onPress  =  function()  {

xMove = button_1._x;

button_2.onPress = function() {

xMove = button_2._x;

button_3.onPress = function() {

xMove = button_3._x;

button_4.onPress = function() {

xMove = button_4._x;


The Whole thing works without this code !!! So, i am just confused…
what is the importance of the code above? :hangover: Can anyone explain me why this code is there in the fla? I mean, i deleted these lines and it works perfectly.

Please forgive me if this is stupid question :+) . I am learning action script and I understood everything in this tutorial except the above thingie. Thanks.