Fills the same niche as Cordova/PhoneGap or Xamarin. It's kinda the best of both. Intuitive dev process for web developers not needing to learn new paradigms. Fully native compilation for better performance than WebViews.
Are you sure they compile to native? IIUC they use JavaScriptCore on iOS, which currently doesn't JIT. Probably good enough to drive UI and you can implement performance hungry stuff in native code.