The perspective of beauty and knowledge definitely changes from person to person. But what about purity? Purity is something that helps us reach a better understanding of the nature of people, it helps us realize the close attachment of people's thoughts, it creates a space for us to think, a space where no thought stains us and eventually, purity helps us reach the eternal in itself.
Purity is something very closely related to spirituality and that is a reason for us to worship those people as God Himself. I feel that's why people say, 'Rare is the unity of beauty and purity.'