Why is extending MovieClip class different?

I’ve been going through the OOP AS 2.0 tutorial here on Kirupa, and I was curious WHY this code is in a class which extends the MovieClip class and is used to affect a linkage item in the library:

// init method for arguments which in other normal
    // circumstances (non-MovieClip classes) would be
    // given to the constructor. Not an option with MovieClips
    function init (pGravity:Number, pBoundries:Object) {
        gravity = pGravity;
        boundries = pBoundries;
        return this;

Is there something undocumented about why I wouldn’t pass those parameters in when instantiating a new instance of the class in question? What is different about extending the MovieClip class?