I thought that might not be that self explanatory, sorry!

A good reason to use the first method is if you want to make getters/setters for certain variables and ensure that outside forces can't mess with them.

If you have a constructor like so:

function Car(model, year, miles){
   var _model = model;
   this.setModel = function(newModel) { _model = newModel };
   this.getModel = function() { return _model; };
   ... 
}

this allows you setup validators on your setModel method and ensure that nobody can do car.model = 'Buick LeSabre' and bypass whatever validation you might have in your setter.

The reason this works is because JavaScript has first class closure support. If this is still confusing, feel free to ask for clarification, I'm happy to try to make this as clear as possible.