Plesioth vs fishing harpoons

I googled and advanced searched and found nothing of relevance to my question, which arose, during plessy quest after supplies got delivered. They give you fishing harpoons. plessy is more or less a fish, okay i think. Ive always just pounded him to death. So what am i missing here with the harpoons? its not like i cant carve him if i dnt kill him with one, does it deal some kind of massive damage or make parts easier to break?

According to the Brady guide it's so you can stab it in the belly if it doesn't let you fish it out at the start of the fight to get a shiny. I haven't tried it out to be honest, but that seems to be a simple enough explanation.

Try asking in the Q&A thread in stickies next time though, this belongs there.