Why do sea otters use rocks when feeding on shellfish?

Answer: A rock helps crack the hard shell and expose the food

The rock scrapes edible algae off the outside of the shell — The otter is reaching the animal inside the shell, not harvesting algae from the outside. The stone provides a hard surface for impacts that crack the shell and expose the meal.

The weight pins shellfish underwater so they cannot escape — The feeding tool is used to break shells, not as a weight to stop prey escaping underwater. Otters can bring prey to the surface and open it there; the barrier is the hard shell.

A rock helps crack the hard shell and expose the food ✓ — A rock can serve as a hammer or as an anvil against which a sea otter pounds shellfish. Breaking the shell gives access to the soft food inside. This is tool use during feeding, not merely playing with a stone.