How does FileVault 2 recovery key stored in iCloud work?
The recovery key absolutely isn’t stored locally by Apple, so you either need a safe place to store it where you won’t lose it or Apple will help store it for you to recover later when you choose to unlock the Mac by reading the key from iCloud. 


in a nutshell, your iCloud password gets you your key back instead of you needing to choose where to store it (paper in a safe, 1Password app, word document, under the keyboard, screen capture of the image of your key on screen, etc...)