They are very useful for getting black pearls, chitin, silcia pearls. They have a traqnquilizing poison and one hit will knock you out. Luckily they can only stay on the ocean floor but sometimes they climb up the floor and end up on land.
If you need black pearls on valguero, there is a cave hidden in the jungle on a hill near green ob that you can enter and drop down a big hole into a water pool. There are 4 or 5 pools in the cave, each with up to 15 eurypterids in them, you just have to kill the eels before you get the eurypterids. And the beauty of this cave is that if you leave it (make sure to bring grapples to get out if you don’t want to go the long way) the eurypterids will be back and you can farm another few hundred black pearls. It’s even easier in single player where you can exit to main menu to refresh the spawns.