Why we use prototypes here?

It would probably look more like this:

Song.prototype = {
    play: function(){
        // code
    },
    stop: function(){
        // code
    },
    toHTML: function(){
        // code
    }
}

Anyway, from what I saw the code looks pretty easy to read so it's probably fine the way it is in a small application.

Also, prototype isn't a function. The prototype of an object is basically the object that it's modeled after. So all those methods (play(), stop(), etc) that you're putting on the prototype, aren't actually on Song. They're on it's prototype object. When you call Song.play(), JS looks in the scope of Song and doesn't find it there, so it moves up to it's prototype to look for it. That's where it sees play().

I think that answer is when you have an application with million of access, you need to start to think about performance, and if you create a playlist that a user click and he create a new instance, you application can be too slow. and also if you think only in client browser thats ok, but when u start to think about put this code in the server (node.js) you can have a problem.