Earlier quoted context omitted.
What’s there to manage? They are files. Read a book, copy a new book over, delete old book. A file manager does all that, and is simpler and easier.
Most (all commercially available?$ebook readers are not as simple as that. And a file manager doesn’t track which books of mine are synced and which are not. Calibre does. A file manager does not make it easy for me to correct the ebook’s metadata so it appears properly on the ebook reader. Calibre does. Calibre will also automatically download metadata for you from a variety of sources (with many options for the lev…
Only if you make them. Have been pretty simple for me. I don’t do most of the things you mention, and do not see myself wanting to.
I do use Calibre for conversion (through the command-line ebook-convert tool, wrapped in a script that converts and mails to Kindle in one operation). I don’t use different devices, to “automatically” does not mean much.