Composition using factory functions and Object.assign()

composition-object-assign.js

var runner = function(){
  return {
    run: function(){
      console.log(this.name + ' is running away!');
    }
  };
};

var swimmer = function(){
  return {
    swim: function(){
      console.log(this.name + ' decided to have a dip in the pool');
    }
  };
};

// Compose an Athlete
var athlete = function(){
  return Object.assign({}, runner(), swimmer() );
};

// Create a Runner
var goose = Object.assign({ name: 'Goose'}, runner() );

goose.run(); 

// Create a Runner and Swimmer
var jared = Object.assign({ name: 'Jared' }, runner(), swimmer() ); 

jared.run();
jared.swim();

// Or do the same thing but with an Athlete
var frog = Object.assign({name: 'Frog'}, athlete() );

frog.run();
frog.swim();

Why factory functions that return objects and not just object literals?