If I use the web interface to my self-hosted library, each book's cover is shown along with a progress bar if it has ever been opened in the web interface.

If I use the OPDS interface, that doesn't happen; I suppose it would be nice to push some reading information back. Sync between reading devices is handled by koreader-sync, so I can pick up any device running koreader and be on the page where I left off.

They do, but a physical book has a presence that digital books lack. Like weight, cover material, print quality. I can learn from digital books fine and read novels on an ereader. But a physical book anchor your memory like no other.