Ionic has no benefits over phonegap coze you need both
First of all Cordova/Phonegap are the basis for hybrid apps with Ionic.
In short words it does give you the UiWebView where your app is loading something like a browser. (soon we can replace it with WKWebView (faster!))
Ionic is a Framework like Bootstrap or Kendo UI etc…
the difference between others and Ionic is, they did concentrate on making a high-performance mobile framework built with AngularJs.
if you want a more detailed overview search for mobile frameworks at Google or Bing
something like this: http://moduscreate.com/5-best-mobile-web-app-frameworks-ionic-angulalrjs/
i hope i could help you.