Non-symmetry

All animals have left-right (bilateral) symmetry, but don't have dorsal-ventral (belly vs. back) symmetry, or anterior-posterior (mouth vs. rear) symmetry, because these other kinds of non-symmetry are themselves beneficial.

Dorsal-ventral non-symmetry (the fact that the belly-side of most animals is different from the back-side) traces back to our ancestors that either swam or crawled with belly-side down and back-side up ... so and the dangers from above are different from the dangers from below. There is no reason this has to be true for we humans who walk upright , there's no reason our belly is soft and back is hard except that this has some benefit for animals that walk on all fours.

Anterior-posterior non-symmetry (the fact that the front of the animal is different from the rear) traces back to our aquatic ancestors, where one end had a mouth, and the other end was for excretion and so there was obvious benefits to keeping them at opposite ends of the animal.

However, there have never been benefits to left-right non-symmetry,  animals are just as likely to get attacked from the left as from the right , or to find food on the left more than the right.