Why is the apply() function needed in the constructor

Question

function Set() {          // This is the constructor
    this.values = {};     
    this.n = 0;          
    this.add.apply(this, arguments);  // All arguments are values to add
}

// Add each of the arguments to the set.
Set.prototype.add = function() {
    /* Code to add properties to the object's values property */
    return this;
};

This is the beginning of the code used in "Javascript: The Definitive Guide" to create a 'Set' Class. I've tried to rationalize the necessity of the apply() function in the constructor but can not figure it out for the life of me.

this.add.apply(this, arguments);

if the add() function is already a method called by 'this' then what purpose or use is the core apply() function fulfilling. Thanks in advance to anyone who attempts to explain this to me

http://jsfiddle.net/Marlin/Ydwgv/ - Full Example from Javascript: The Definitive Guide

Answer 1

"if the add() function is already a method called by 'this' then what purpose or use is the core apply() function fulfilling."

So that the arguments can be passed on as an entire collection. Useful if Set accepts an indefinite number of arguments.