The major benefit is using KOReader instead of Amazon's reader. Main benefit of that is configurability in my opinion. I'm new to full-time use, so I'm not aware of everything it's capable of, but it's already way better. My headline features are direct connection to Calibre over the network, and the lack of any marketing to me whatsoever. My nice-to-haves are arbitrary lockscreen images, Wallabag support (have yet to explore this, but looks real fun), and the built-in RSS reader.

Other than that, jailbreaking gives me the ability to fully disable ads and OTA updates.

Everything else is pretty much toys and eye candy. KOReader is the main thing from what I can tell.