Currently seems like a novelty tool, back when you could harpoon bosses you could drag them out to sea or into a hole and just go to town with a bow. You can still use it to get serpent scales from sea serpents but most people seem to dislike the serpent scale shield. (I love it simply because it looks cool and if nothing else can put a few of them up in my den for some sweet feng shui. This also goes for the harpoons themselves.)

There are some niche uses like dragging golems into holes but it isn't the most consistent thing in the world. After spending hours hunting probably close to a hundred golems, I can say that it's usually just not worth the trouble.

It would be nice if they reworked the harpoon to be compatible with fishing or to make it a strong tool like it used to be. Or, at least, give us a few harpoon upgrades so it can be a viable weapon!