Well nowadays Nova isn't running on top of the launcher, it's running INSTEAD of the default launcher. But I can't see how it will save more battery other than it turns off access to certain battery hogging features like the Daily Briefing, but you can turn that off without using a different launcher. And enabling more features via Nova will accordingly have its own battery drain.
Mainly thr benefits of having Nova or any third party launcher is added customizations and features, which still differ depending on the launcher you use. Also, Nova launcher supports unread badges for all apps, not just the few supported ones by Samsung.
Gestures are also customizable. Like swipe up fot app drawer, double tap to lock, double tap swipe for another app, two finger swipe down for another, etc.
Added functionality on the launcher is also possible. For example, instead of opening Spotify then going to library then playlists to choose your playlist, using Nova Launcher Prime with the Sesame Shortcut addon, a long press on the Spotify icon will bring up a list of your predefined playlists.
But these aren't visible really. Most visible are the changes in the look of the phone. For example, this is my S7Edge homescreen:
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