why do otters save rocks?

Otters save rocks primarily for utilitarian reasons: they use them as essential tools to help them eat.

Because otters don't have sharp claws or strong enough jaws to crack open hard-shelled prey—like clams, mussels, or crabs—they rely on these "favorite" rocks as anvils. An otter will float on its back, place the rock on its chest, and repeatedly bash the shellfish against the rock to break it open and access the food inside.

Here is why they keep specific rocks:

• Efficiency: Finding the perfect rock takes time and energy. Once an otter finds a rock that is the right size, shape, and weight to be effective, it becomes a prized possession.
• The "Pocket": Sea otters have loose, skin-covered pouches under their forearms (essentially a fold of skin). They use these pockets to store their favorite tool while they are diving or swimming, so they don’t have to search for a new one every time they want to have a meal.

In short, they aren't collecting them for sentimental reasons; they are keeping a specialized piece of equipment handy to make their daily foraging much easier.