I happen to like the <kbd> tag for showing when keys should be pressed, but I can never remember it on the very few occasions that it would actually be useful. Having it in the help file would mean that I don't need to find another question that has the tag in use, and try to copy it.
I know that some people do crazy things with this tag, but in general I think that it is useful.
Assuming you are referring to this Help page, it is currently mentioned:
Inline HTML
If you need to do something that Markdown can't handle, use HTML. Note that we only support a very strict subset of HTML!
To reboot your computer, press <kbd>ctrl</kbd>+<kbd>alt</kbd>+<kbd>del</kbd>.
This is not very explicit, though, and can not easily be found when you don't remember the tag to begin with. Adding an additional sentence along the lines of "Highlight (keyboard) buttons by enclosing them with the <kbd> and </kbd> tags." would improve that.
The tag is also mentioned in the linked list of allowed HTML tags.
The "new" editor (which you can opt into through the Settings on your profile) adds a drop-down menu through which you can access some of the markup tags (super/subscript, spoiler, meta, and keyboard (and the shortcut Ctrl+' for that last option)).
This script will add similar functions to the toolbar of the old editor, as well.
