This part from the release notes pretty much summarizes what this feature is:
The User Switcher panel applet (package fast-user-switch-applet) now provides an extra entry for starting a Guest session. This creates a temporary password-less user account with restricted privileges: the account cannot access any users' home directories, nor permanently store data. This is sufficiently safe to lend your laptop to someone else for a quick email check, or to use it as a surf station for guests in your house.
It is a nice implementation, and a nice security idea to not show it at login, but they are thinking about it, if the Ubuntu dvelopers can contact the GDM developers.
Use cases? Well, one user on Brainstorm explained his opinion on this issue clearly and directly:
Its good because sometimes my sis or mom nag on me to use the computer.
And I don't want them to find my porno, or accidentally delete my files.
And having the guest account get purged on logout, make me so I don't have to have their crap on my system when they logout.
Looking forward to everything in 8.10. :D
UPDATE: There is another way to make a guest account. This is better, in my opinion, in case you want the guest account for your family and you want them to be able to save their work, and you want to place some icons on the desktop for easy navigation. (Thanks to Nathan, in the comments!)
Go Applications → Accessories → Terminal, and type in:
- sudo adduser NAMEOFUSER
- sudo passwd -d
- sudo gedit /etc/pam.d/gdm
- auth sufficient pam_listfile.so sense=allow file=/etc/passwordless item=user
- sudo gedit /etc/passwordless
UPDATE 2: I have finally found a screenshot of the guest option in the User Switcher applet. Thank you, "Admin"!