It isn't needed, unless you want it. The features of kepubs include:
- Popup for footnotes
- Can zoom images
- In-book progress stats
- Slightly faster page turning (but not fast enough that I notice it)
The features of epubs include:
- Uses Adobe page numbering
- Supports rendering features such as ligatures
Note that I have deliberately used the word "features" rather than pros and cons. I'll let others make claims on as to what they are and which format is "optimal".
But, overall, if you don't read books with images and footnotes, there are not that many differences. If you read those sort of books, then kepubs have a distinct advantage.
Honestly, the best thing is to try it and see what you like. As you are a calibre user, install the KoboTouchExtended driver. That transforms books to kepubs as you send them to the device. Try a couple of epubs, try a couple of kepubs and see which you prefer.