Why do we use Closure in Javascript?

I've used closures to do things like:

a = (function () {
var privatefunction = function () {
alert('hello');
}

return {
publicfunction : function () {
privatefunction();
}
}
})();

As you can see there, a is now an object, with a method publicfunction ( a.publicfunction() ) which calls privatefunction, which only exists inside the closure. You can NOT call privatefunction directly (i.e. a.privatefunction() ), just publicfunction().

Its a minimal example but maybe you can see uses to it? We used this to enforce public/private methods.