Portability, isolation, the ability to run pretty much anything inside. They do consume more resources, but if they’re that much slower then there’s probably something wrong in your setup.

I’ve tried it a few times, never stuck. I guess it’s just convenience, it is a well integrated piece of software, especially if you use both LXC and VMs. Personally I keep using virt-manager and Cockpit.

I’m not sure if any app syncs progress with eReaders, but I haven’t looked into it. Maybe KOreader can? Other than that Audiobookshelf works well with ebooks. I haven’t tested it with graphic novels, but it does handle regular ebooks (PDF and EPUB) just fine.