What role does the user@host that often appears at the end of a public ssh-rsa key play? Is it necessary? Does if serve any purpose in the authentication, or is it simply a record of who and where the key was created by for the information purposes (for a human reader)?
It's merely a comment. You can put anything you'd like there. Or nothing at all.
Most key generators put user@host there to help identify on what host the key was generated, and for which user.