Taking 'super-user' to mean 'root' seems like a big assumption. Is there anywhere in the Unixverse where root is specifically referred to as 'super-user'? I sort of thought 'super-user' would refer to any user with higher privileges. Well, really just different privileges, but why would you switch to a user with lesser permission to preform the action you probably just got denied? I assume the presented code was just that way because it covered 99% of use cases and was good enough for then and that root is now the default user for the same reason.
I'm not 100% sure where you're going with all that, but there are legitimate reasons for the super-user to "move down" into a specific user's identity -- for example, to be sure that files/directories created during some maintenance are owned by that specific user, and have that user's umask applied. Another instance would be file cleanups when the super-user specifically didn't want to be able to delete arbitrary files. There are lots of other cases.
