For example, If you want to boot from something else, e.g. a CD, than, you'll need this.
So let's get to it, it's really simple.
1. Press [Windows key + R] on your keyboard (This opens the "Run" box)
((Sorry, my system language is Hungarian, ignore that))
2. Type: "cmd" (Wihout quotation marks of course)
3. In the new window, type: "shutdown -s -t 00 (Without quotation marks of course)
(("-s" goes for shutdown type, if you type "-l", the system will log out for example; 00 is the time before shutdown))
4. You are done The system should nicely shut down now.