Not desperate, if one is slow to vacate then provided a long list of conditions are met (no adverse braking action, daylight, pilot of lander can see landed aircraft....) a Land After instruction can be issued. This simply gets it out of the way to allow the controller to move onto other things and stops the pilot getting itchy with the TOGA switch when he doesn't get a landing clearance till over the threshold. The main point being is that it is not a "clearance to land" but that the controller has now put the decision into the hands of the pilot, if the pilot is happy then he lands, otherwise he goes around or hangs on for a last second standard landing clearance. Clear as mud?