It creates a epub out of any site. And if theres a next button, it pulls from that as well. So a "book" is produced from many different sites.

Theres a lot of people that use forums/wordpress/comics/etc.... for their content of choice. But hey I want to read it on an ebook reader. So I use the extension boom epub.

Also its all in one file so you dont have to hunt and peck for resources. Its really useful on say royal road or sites that dont have a https://archiveofourown.org/

And it says offline in your library.