Don't understand why JSX Harmony support commonjs so good!!!

I'm on Mac, and start to use WebStorm from WebStorm 6.
Recently, I've noticed a great improvement!!!

It's to switch the Languages & Frameworks -> Javascript -> Javascript language version to "JSX Harmony",
webstorm will auto-suggest the node modules installed in the current project while I am typing require("...").

It's great to avoid typo error to require node module, no matter it is node module or local file module.

However, it seems this feature is only enabled in "JSX Harmony",
because I've tried ECMAScript 6 , or JavaScript 1.8.5 ... etc. all of them have no this feature.

Why ???!!!
I don't understand....