Oleg asks "Why does one need to logout from Forms?" Let's put it in another context: "Why does one need to logout of Hotmail?" "Why does one need to log out of Facebook?" If you can log in, shouldn't you be able to log out? In an environment with shared computers, where the user of a given computer changes every hour, it is a requirement that the next person to use the computer no longer has access to the previous user's data or account. Think of a kiosk scenario, or in our case, a school with a cart of laptops that is rotated among classrooms and students throughout the day. Even aside from the opportunity for abuse and data loss, there is the issue that when a form such as a quiz is configured to only allow a user to submit it once, when the next user comes along to try to take the quiz, even though they are logged into their Office 365 account, when they open the form it is still logged in as the previous user. And there is no way to log them out of Forms other than clearing cookies and cache. The user doesn't know this: all they get is the message that they have already submitted the form (not true, that was the prior user), and there is no indication of who is logged in to Forms or any controls to log them out from the Forms page for the quiz.
Microsoft