A question about actionscript

[color=black][font=Arial]I was going through a tutorial, which only gives the script and no explanation, and I ran across something I don’t quite understand. I am sure that it is a duh moment for me but here goes and any input would be greatly appreciated.[/font][/color]

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[color=black][font=Arial]This snippet of code makes sense to me mostly all except for the highlighted portions. [/font][/color]

 MovieClip.prototype.reset = function() {
	// specify the width and height of the movie
	width = 550;
	height = 200;
	// -------------------
	var dist, norm; // Here we declare the local variable
	this.x = this._x;
	this.y = this._y;
	this.speed = Math.random()*4+2;
	this.targx = Math.random()*width;
	this.targy = Math.random()*height;
	// here we set them
	dist = _root.getdistance(this.x, this.y, this.targx, this.targy);
	norm = this.speed/dist;
	// 
	this.diffx = (this.targx-this.x)*norm;
	this.diffy = (this.targy-this.y)*norm;
};

[color=black][font=Arial]My question is why do you declare it then set it. Doesn’t it automatically declare it when it is set? [/font][/color]

[color=black][font=Arial] [/font][/color]

[color=black][font=Arial]Again I am sure it is a duh moment so thank you for bearing with me! Oh and I am very new at this whole actionscript thing.[/font][/color]