Synthesizing functions in javascript... practical applications?

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From David Laing & Greg Malcolm's excellent javascript koans. I'm wondering: What are practical uses cases for this function synthesizing in javascript, where this approach would be preferable to all other possible approaches:

  it("should use lexical scoping to synthesise functions", function () {
    
    function makeIncreaseByFunction(increaseByAmount)
    {
      var increaseByFunction = function increaseBy(numberToIncrease)
      {
        return numberToIncrease + increaseByAmount;
      };
      return increaseByFunction;
    }
    
    var increaseBy3 = makeIncreaseByFunction(3);
    var increaseBy5 = makeIncreaseByFunction(5);
    
    expect(increaseBy3(10) + increaseBy5(10)).toBe(28);
  });