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.

Heck, I remember telling people over IRC chat they could get more features from the Alt+F4 menu.

Don't you mean Jon Arbuckle?

Quite some time. At least since the first Steam Controller, 2015. Possibly before that.

it's a fairly good idea, ignoring the funny outcomes caused by allowing them to have "shut down" be an option.

Nermal is literally just a kitten who uses him as an insult.

Dev probably had to report it. I could see getting bad review bombed for people unaware of

It’s not OS specific. It’s through Steam’s Controller Profiles which has a Shutdown hook to Windows and Linux distros