The end product of a Chromecast is you get to watch something you have selected from YouTube or Netflix on an iPad or iPhone or most other similar gadgets and watch it on your HDTV.
You can already do that with your Rokus. So the only advantage with a Chromecast, assuming you have the supporting gadget, is a much easier search since you have a keyboard and, in the case of the iPad/iPad, voice input. So finding things is quicker. That's really it.
Also, it always makes me nauseous to watch Google employees give one of their marketing pitches (just watched the one for Chromecast on Youtube). Just dripping with "We are going to make your life better for you, ON OUR TERMS AND WE DEFINE BETTER".