Without it means for most book you will not be able load the book on a non kindle reader.
I think it's because I download the file for my old Kindle Keyboard 3G, so I probably get a file without DRM. Not sure. Calibre "just works".
"In addition, the owners of older e-ink Kindles running firmware prior to 5.6.5 (August 2015) can access books in non-KFX formats directly from the device via USB."
Don't lose it, might try to buy on old kindle :D
Edit: I don't really care if I lose it, though. I just won't tell Amazon if I do.
I think it's because I download the file for my old Kindle Keyboard 3G, so I probably get a file without DRM. Not sure. Calibre "just works".