Whenever I try to research this question the answer usually presented is along the lines of "so the outside world does not have direct access to the local variables." What's missing here for me is the context to which this applies. I get that, for instance, in the code below 

```
function Person(firstName, lastName) {
  var _firstName = firstName,
  _lastName = lastName;

  this.firstName = function(value) {
  if (!arguments.length) return _firstName;
  _firstName = value;
}};

```

the variables are considered private because they can only be accessed from calling the functions. What is the significance of this? Why does it matter? And most achingly mysterious is what is a "real life" situation in which the common explanation of "so the outside world does not have direct access to the local variables..." would apply/make sense to someone who has not seen a situation where it matters.

Thanks SO.