It's a library manager, like iTunes for music, or Plex for movies, Google Photos/Picasa for photos/images . You pick a spot for you library locally, and then your local lib is a jump off point to load in on to any reader device you want. It will understand what device you are pushing it to, and automagically convert it (like Amazon’s proprietary format to mobi or epub 😜 !) to supported file-types. If you are into that kind of stuff, you could run it as a service on your network, and have all that fancy BYO cloud ebook solution.

The big difference with just copy-pasting is that you have a full library somewhere locally, and you can pick and choose what you load up on your reader. For me and maybe you, those lists are pretty close to identical, but what if you have a very large collection? And what if i just had to RMA my Libra? One click and a couple minutes after i receive my replacement, all of my books and reading progress will be synced back. If you had put your lib on the device itself, you would have had to rebuild it from scratch.

TLDR: Collection Management/Self Host and auto-convert are the big plusses.

It's like iTunes, but for books.

My kindle has been on airplane mode for years and I read new books all the time with it, but hey, whatever works for you