What are the features or functionality which is present in Rational Application Developer and not in Eclipse? Why is Rational Application Developer needed?
By default Eclipse is only Java development environment not Java EE, so to develop for Java EE you need Eclipse bundle for Java EE developers. RAD is based on that, plus has many other features more strictly related to developing and deploying applications to various WebSphere products.
Here is short list with additional features in RAD (it is not complete list and might change in detailed comparison between specific RAD and Eclipse for Java EE versions):
You can see whats new here - RAD 9.1 new features and enhancements
If you plan to buy WebSphere Application Server, you might be interested in the Tools Edition license, where in addition to server licenses you get unlimited number of RAD licenses for development for that runtime.
UPDATE
If you just need server support in Eclipse there is WebSphere Developer tools plugin for Eclipse, freely available via Eclipse Marketplace, which supports WAS 8, 8.5, 9 and WebSphere Liberty.