In the case of menubars the default will be horizontal but it's possible people will want vertical ones. This orientation attribute should control which arrow keys are used for navigation as well as setting the aria-orientation value. It should probably also apply some default styles to actually make the menubar render vertical.

MDN suggests you'd also want this for menulist with a default of vertical, I'm not sure if this is actually useful for menulist and what the rendering would look like but for the sake of completeness it seems reasonable to support both.