Martijn
kbd's name is definitely from "keyboard", but the current spec describes it as a way to mark-up any user input. That would include selecting menu items and the likes. It goes well with samp, an element that represents computer output. This isn't going to break validators though. I broke those using main elements, because WHATWG and W3C have slightly different interpretations of those.
Nkrs
It was initially for displaying keyboard input, like CTRL+ALT+DEL, but I think they expanded its use for other inputs as well. It could be a useful hook so that screen readers would expand CTRL into "Control", but I have no idea if it works that way.