It's just that compared to Calibre, this software offers less editing or library management functionality. On the other hand, it seems more pleasant to the eye for PC reading, especially for comic books.
I just installed FBreader to run the test. Main windows highlights the book covers, it's more user-friendly. Reading is easier. There's a built-in dictionary.
you can take a look at this article, everything is there: https://www.omgubuntu.co.uk/2018/03/lector-qt-ebook-reader-app-for-kde
As this task has been marked Needs Maintainer for a month with nobody having stepped up to become maintainer, in addition to providing an acceptable patch for inclusion, marking as WONTFIX. Feel free to re-open when a patch has been submitted via the proper processes.