Bilateral (humans) or radial (jellyfish) symmetry helps keep you balanced so you can move with purpose, and reduces the needed genetic complexity.

You don’t need totally separate developmental pathways for your left arm and your right arm, just a genetic copy-paste.

You’ll just swim in circles with one large fin and one short fin.

Both the vertebrates and the arthropods settled on bilateral symmetry long ago, so practically all land animals are symmetrical like that.

Radial symmetry is more popular among the jellyfish/anemone/sea urchin types in the ocean.

Asymmetric anarchy like bivalves and sponges still has a few very ancient holdouts.