What is the purpose of logging out after WordPress upgrade?

I had recently noticed when testing WordPress 4.0 upgrade, that it logged out of the site and went to the login screen, with a message saying to log back in to enjoy the new improved WordPress.

What is the reason for this logout? I don’t remember seeing this previously, and when a programmatically-called WordPress upgrade occurs the user isn’t logged out and it works fine.

4.0 made a change to the structure of the authentication cookie and added a “token” feild to it. I assume that you were logged out because your older style cookie didn’t match the new format.