Why wrap a helper function in an IIFE instead of defining it inside the body of the function using it?

I was playing a bit with Babel and ES6, transpiling some code and I was stucked at this part:

class App extends SomeParent {
    myFunction() {

    }
}

The output that I'm interested in, is this:

var _createClass = function() {
    function defineProperties(target, props) {
        for (var i = 0; i < props.length; i++) {
            var descriptor = props[i];
            descriptor.enumerable = descriptor.enumerable || false;
            descriptor.configurable = true;
            if ("value" in descriptor) descriptor.writable = true;
            Object.defineProperty(target, descriptor.key, descriptor);
        }
    }
    return function(Constructor, protoProps, staticProps) {
        if (protoProps) defineProperties(Constructor.prototype, protoProps);
        if (staticProps) defineProperties(Constructor, staticProps);
        return Constructor;
    };
}();

My question is why they used this way/method for _createClass var of using IIF and then returing another function instead doing something like this:

var _createClass = function (Constructor, protoProps, staticProps) { 
    function defineProperties(target, props) {
        for (var i = 0; i < props.length; i++) {
            var descriptor = props[i];
            descriptor.enumerable = descriptor.enumerable || false;
            descriptor.configurable = true;
            if ("value" in descriptor) descriptor.writable = true;
            Object.defineProperty(target, descriptor.key, descriptor);
        }
    }
    if (protoProps) defineProperties(Constructor.prototype, protoProps);
    if (staticProps) defineProperties(Constructor, staticProps);
    return Constructor;
 }

without having to use IIF and return another function...

Is there a good reason/practice?

Link for babel demo here

Babel's current output will only create the defineProperties once wheras your example will recreate the function for ever single class declaration.