Optional but on by default since it benefits most users. What I don't like is that the setting that controls it is not obvious or really noted, and it's tied to another setting (so they are either both off or both on). To disable, go into the Settings, then Accounts, then Sign-in options. Toggle off the Privacy option that says "Use my sign-in info to automatically finish setting up my device after and update or restart". Another side effect of this setting being on is that whenever you reboot, the account that initiated the reboot will automatically have logged in session after a reboot. So for example, in my family we have 4 user accounts on the main desktop - Lets call them U1, U2, U3, & U4. If I am user U3 and initiate a reboot, when the system comes back online U3 automatically gets their profile logged in (you don't see this, it just happens in the background and you still have a logon screen you must authenticate through). So Imagine U3 initiates a reboot, then leaves since they don't need to do anything more, then user U2 comes by and logs in, there will be 2 user profiles/sessions active - the U3 which auto logged in after the reboot and the manual U2 logon.