Why do we give so much importance to the symmetry of face in the opposite sex? Why do we yearn to reproduce with a person with a beautiful face? How is it important for survival, if at all? Is there a biological explanation for this?
Parasites, disease, or injury can cause asymmetries. The complexity of the face, and our visual sensitivity to facial features, makes us most sensitive to subtle symmetries and asymmetries in facial features.
Therefore facial symmetry is a sensitive and reliable indicator of health. Healthy mates are more likely to possess good genes to pass on for strong healthy offspring. Healthy mates are also likely to be more physically capable of producing and raising offspring.
Any individual who possesses a gene that causes attraction to symmetric partners is more likely to select such a mate. That individual is therefore likely to produce more offspring, and more likely to produce strong healthy offspring.
Therefore the gene for attraction to facial symmetry is more likely to survive and spread.