In an post from Aaron, he explains RunAs and it's use. Following points are covered:
What is RunAs? How to use RunAs from the GUI (even if you can not see it). Using RunAs from the command line. When RunAs will not work. Useful RunAs shortcuts and related tips for the non-admin.
I think he makes a good point for all those users who work as administrator on their home PC's, connecting to the Internet and not knowing the consequences by doing so. By logging on as a simple user, and working with RunAs, you can do most administrative tasks without the risks of using the administrator account constantly.
To read the post, go here.