How it works: You've got a friend (well, probably more than one, but for the purposes of this example, just one). Your friend, a curious and look-through-your-medicine-cabinet sort, needs to check his email and get some driving directions on your computer. Don't want to give Nosy McSnoopsalot free rein to peek through your collection of model railroading websites? Just log in with the Guest account; a fresh, unimpaired new user for temporary access to your machine. As soon as he logs back out, the Guest settings vanish into the ether, and the next Guest who logs in gets the default settings, desktop, etc. as if the original guest had never been there.
Who will use it: Sooner or later, everyone, unless you have no friends -- or you don't care who gets their grubby mitts into your stuff.