I’ve only just installed it this week, but for me:

- DRM support (via a Plugin)

- Dark mode on an older device

- SSH Server

- Custom dictionaries

Next on my list is to find a plugin that can transform imperial to metric for when I’m reading American books

Edit: oh and also something that syncs my progress on books not bought from Amazon on multiple devices

there is even a Syncthing plugin: https://github.com/jasonchoimtt/koreader-syncthing