It's a webserver, file manager, sync client, transcoder/converter, RSS reader, metadata manager all in one.
With all possible regards and respect to the author, I find calibre a terrible mess. It does a lot of things, none of which very well. And the one thing that I want it to do, it doesn't. Not reliable or not at all. I want it to give me access to the ebooks I buy online.
This isn't calibres' fault, but the industries'. DRM, incompatible formats pushed by tech giants. But in the end, all I want is a way to see, reference and sync the books that are on my Kobo with my (Linux) desktop.
A python script, A proprietary Kobo app over wine, some bash scripts and a well kept directory achieves this. But calibre doesn't. Though it can read RSS feeds.