What is the point of prototypes in JavaScript? Why not add method directly to constructor?

So I am going over a few lessons over JavaScript at CodeAcademy, and I ran into this section regarding prototypes.

A bit from the code:

function Dog(breed){
    this.breed = breed;
};

Dog.prototype.bark = function(){
    console.log("Bark!");
    };

I don't understand the point of doing it that way. Why not just do it this way? (The way I first learned.)

function Dog(breed){
    this.breed= breed;
    this.bark = function(){
        console.log("Bark!");
    };
}

The second way, everything is together in one block, and not separated. What is the advantage of the first one? I know that there is, I'm just not seeing it.