Obviously not a reason to attach a Qtag to a carry on bag anymore (or maybe I'm hallucinating and it never was) but I thought I remember long ago when Qtags were introduced, they could provide easy and quick access to lounges? Also let Qantas keep track of you if they ever needed to track you down? Of course that'd necessitate Qtags hard-coded to the owner which isn't the case so I don't think I'm correct.
Location tracking would be useful though. PA announcement: "Can Mr. Smith please hurry the f up and finish eating your McDonald's and proceed to gate 3"