Comment 4 for bug 550502

It would be useful if, for instance:
Your child is playing, and an update manager window appears; you don't want him to install updates, so you need to provide your password for that kind of operations. But then, you don't want to be prompted for the password every time when you are doing several administrative tasks, so the system remembers for some time that you 'got hold' of super-user previleges. It will expire after a few minutes, I guess. That is indicated by the presence of the key icon in the status bar.
Now, when you are done with some administrative tasks, and your child resumes playing Tux, you'll want to discard those privileges, so he won't do anything wrong by acident. That's what the 'drop elevated privileges' means.

It probably should have a 'What is this?' option in that menu... and perhaps 'Administrative previleges' would be more familiar to window users...

One problem is: it appears when one authenticates: 'Provide your password to allow this or that'. But when one authenticates, he is doing some task (that is interrupted by that prompt), and doesn't notice the icon in the status bar. When latter he notices it, it's not easy to understand where it came from.