It's for people who use Steam in Big Picture Mode as a console replacement, so that they can shut off their PC without getting up from the couch.

Actual answer: steam provides the flexibility to rebind basically whatever you want to a controller input in order to make abstruse keyboard/mouse commands available via controller. It's less that there's a "shutdown computer" option in a dropdown menu and more that some ingenious prankster realized they could bind the windows shutdown keyboard shortcut.

Gives me flashbacks to Goldsrc dedicated server operators sending eject CD or Play CD commands to clients which was possible for some reason.

Clearly you aren't trying to hide hardcore porn games from your family and don't need the ultimate panic button

Seems like an option that could be useful for accessibility controllers.

Why should we cater to people who blindy download controller layouts?

Computers are powerful machines. Learn to use them properly.