The pole piece of a loudspeaker pushed to its limits gets very hot. The air rushing back and forth in the vent is not heated much at all, but the magnet gets hotter and hotter, with the center pole and front pole piece getting the hottest. The air inside the cabinet can become hot, in which case the air surrounding the voice coil is hot too. So the idea is to transfer the heat into some kind of heat exchanger and get it out of the box.