What's The Purpose of Prototype?

This is what FCC says:

Since numLegs will probably have the same value for all instances of Bird, you essentially have a duplicated variable numLegs inside each Bird instance.

This may not be an issue when there are only two instances, but imagine if there are millions of instances. That would be a lot of duplicated variables.

To this problem, I need to write Bird.prototype.numLegs() = 4. However, I thought the following code would eliminate the need to repeat yourself:

function Bird (name, color) {
this.name = name;
this.color = color;
this.feet = 2

var parrot = new Bird ('Bill', 'multi-colored') 
var hawk = new Bird ('Death', 'black') 
var blueJay = new Bird ('Tom', 'blue')
console.log(parrot) // -- > Bird { name: 'Bill', color: 'multi-colored', feet: 2}
console.log(hawk) // -- > Bird { name: 'Death', color: 'black', feet: 2}
console.log(blueJay) // -- > Bird { name: 'Tom', color: 'blue', feet: 2}

As you can see, I don’t have to repeat myself saying that I want every Bird to have 2 feet, since I didn’t give it a parameter. As FCC said though, is that in order not to repeat myself I’m going to need to use .prototype() to do this. Why is this the case? I didn’t have to repeat myself in that last section of code.