The advantage is you don't have to install anything. Whatever books you upload are simply stored in your browser (might be an issue on iOS). If you want to sync, you login into your Google Drive, and books and positions are synced via it.
It is not correctly set to be a PWA, but if you're offline and the tab is still open, it will continue to work.
We have download support and "To Read" might be a collection or a readlist, so you could just download those series onto your device of choice if you don't want to user our web reader. If you use the web reader, of course, progress is tracked for you.
If you're talking more like how Plex does sync, I literally just tried that with a Progressive Web App last week, but it didn't pan out. There wasn't enough control with a PWA (which means I'm going to have to write something native-ish in the future).
If it could download "to-read" books/manga onto local storage and then sync page numbers when you connect to wifi that would be amazing.