You must be thinking we're daft, don't you? You've just put a case on your luggage carrier and now here we are, talking about a luggage carrier again to mount on top of that case?
That's right. Because on a motorcycle you have to make use of the little amount of room you have. Space is scarce on two wheels and putting a tent or a luggage roll in a case would take up too much space. Space you can put to better use.
By putting that tent or luggage roll on top of that case you'd be left with a whole case to fill. Suddenly, a luggage carrier for top cases does not sound so crazy anymore, now does it?