Physicists use symmetries to restrict possible theories of fundamental particles. As an analogy, consider the human face. If you were able to see only the right side of a person's face, you would still be able to guess what the other side of the face looks like, because of the symmetry that our bodies possess. The other side of the face cannot look like just anything, you know what it looks like-because of symmetry. Similarly, symmetries observed in nature limit what theories look like.