If you plan to keep some at your base, don't rely on your feedig trough... Rexes apparently eat fish (before meat), and eat a LOT. Couldnt figure out why mine were dying until I realized my rexes were eating all my otter's fish.
Ptters are good for getting stuff in the water like pearls, black Pearl's and other stuff sometimes. They are also good for holding artifacts, and for keeping you warm in the snow biome or in the cold.
They can carry multiple artifacts, even on official, and provide hypothermic insulation against the cold. Literally a must have for running artifacts if you plan to spam run boss fights for element. 10/10 would smash again.