I’ve been converting to KEPUB[1] for years and two things I noticed immediately as I started doing so and still stick in my mind are more accurate page numbers and faster page-turning performance. Can’t really say anymore how much of a difference it is, since I always convert.

[1]: https://pgaskin.net/kepubify/

Some ebooks that have extremely long chapters have very slow page turning performance towards the end of the chapter. (e.g. most of the Terry Pratchett Discworlds that are essentially one large chapter)

There's probably an O(n^2) page break algo there which is fixed with the kepub format

It can make a huge difference in performance and batterylife, as well as how well images are rendered. I learned this by chance through this (great) site https://standardebooks.org/help/how-to-use-our-ebooks#kobo-f...

This was after a year or two of suffering through bad performance and a janky experience. Except for the purchased books of course. Somehow I never thought it was possible to fix.

The books I buy from the Kobo store always have the page numbers match up with the rendered pages. The books I... acquire through other means... do not.

I haven't tested this yet but kepub imbeds progress data, which means this can probably be synced with Calibre. This is very nice for those of us which keep a larger library and change readers over time. Prior to this, we had to use a very janky workaround to sync this data. Most of us just didn't bother, so when moving books around, the progress data would reset.

Footnotes open in popup windows over the current page instead of teleporting you to the end of the book, is one feature that I love.

I think that for some reason they use different renderers, and the kepub one consumes less battery.

I think you can get more format options with kepub, also page numbers are more accurate?

There’s some extra stats when you open the book on the device. I _think_ that’s pretty much it though. AFAIK KEPUB is just ePub with bits of added metadata.