I can’t think of a feature that KOReader provides that isn’t already in the Kobo software.

There are many many such features, such as

• style tweaks where a book’s CSS can be altered,
• plugins
• an in-depth progress and session tracker,
• the ability to ghost pages (like copyright and index) so they are skipped and don’t contribute to progress stats.
• RSS reader

The killer feature for me however was in-page footnotes, so they show up at the bottom of the relevant page rather than having to trigger a popup or follow a link.

It also supports wireless transfer from Calibre-web, direct download via OPDS from a Calibre server or online providers like gutenberg, or syncthing.

The problem is, as you’ve experienced, there are so many tweaks and features that it takes a long time to get to grips with the possibilities.