As we have the balloons to build airships, we need a way to tie them down, anchor them to the ground but without having to completely land them. After you have shot the harpoon, you could reel it in or out. You could also harpoon an escaped airship from the ground and pull it down.
Maybe you could make it more generic and allow more uses for it. The old Lego kits had like firehoses and such made of rope that had a reel at the other end.