Recently I was wondering why my iMac is showing a Guest account on the login screen:

I’m pretty sure that I’ve disabled it and indeed when I checked it still was. And even more weird, it’s not possible to enable it. Why is that? Even as a fairly new Mac user I know that Apple usually is looking after details and tries to be perfect. The riddle is easily solved. The checkbox for enabling the guest account has a tooltip stating

That’s reasonable. I’ve enabled the hard disk encryption and it would be rather useless if a guest account could decrypt it. But why is there a login available anyway?
Turns out that for the guest login a Mac boots into a restricted environment where only network is working and Safari can be used to browse the web. Nice.

Apropos detail. If you have enabled the “Find my Mac” iCloud function, which allows you to locate a Mac (or iPad or iPhone) remotely anywhere in the world and optionally wipe it clean, you can still use it when somebody is using the restricted guest login.