I'm a Mac user who uses a Kindle and, until I downloaded this plugin, I had no way to open drm protected books which had been stored in Calibre. Leaving them in "My Kindle Content" for Kindle's desktop ereader to open was not much of an option because (despite how good of a library manager the Kindle, itself, is) the desktop reader is nothing more than an electronic way to thumb through hundreds of books until, hopefully, you find what you're after.
This plugin makes Calibre so much more useful by allowing me to not only read drm protected books with my desktop Kindle software, but, also, manipulate other books in my library with Sigil, Skim, etc. - without needed to drop out of Calibre to do so. Thanks a bunch.