The basic laws of physics and chemistry are not repealed when we put up a building. Radial symmetry means the animal can be folded along any plan into mirror image halves. Like a wagon wheel. Bilateral symmetry means that there is only one plane that can divide the organism into mirrored halves, like a wagon. The Phylum Cnidaria is a large group of organisms that are sessile for all or part of their existence.
Sea anenomes, for example, or coral polyps live out their lives attached to the same spot.
Radial symmetry in these organisms is probably a fundament adaptation to a sessile existence. Your awareness of your environment is omnidirectional. You can sense and get food from any direction. The down side is that when danger threatens, you've got nowhere to go. Cnidarians solved this problem by evolving a variety of stinging cells, loaded with nasty little microscopic harpoons, which they can use to stun prey and attack predators.