As someone who thought the same (stock was enough for me) until I had some free time and switched, here are some things I'm really enjoying, in no particular order:

- Format support: EPUB, CBR/CBZ (comics, graphic novels), PDF

- Dark mode

- Fully programmable gestures and shortcuts

- Automatically syncs with my readeck instance (think readitlater/instapaper) to dowload the articles I save for example here on HN

- Calibre integration over wifi

- RSS/Atom

- Custom dictionaries

If you are into manga, there is an extension for directly downloading them on the device

I'm really happy I made the switch.