Whats the need and use of nested functions in JavaScript

I understand what a nested function is, but I don't understand why we even need nested functions in the first place. Is there a problem that can only be solved by using nested functions in JavaScript. All the examples I see which create nested function, can be coded without creating a function inside a function and will result the same. So which problem requires creation of nested functions, and can only/efficiently be solved by using nested functions.