In Tamil language Kalyana means marriage and that is why the complete word means the holy or beautiful marriage. This goddess is the guardian of marriage and the goddess is another incarnation of Devi Parvati, the wife of Lord Shiva.
So, Goddess Kalyanasundari is still worshipped with the hope that she will make the marriage strong, lasting, and full of love and will help the couple to enjoy the conjugal life for a long time. As she is the guardian of marriages and the conjugal relationships, no marriage can be sanctioned in the eye of God without the worship of Goddess Kalyanasundari. So, if you are in South India and you want to get married in the South Indian style, then you have to go through this worship of the Goddess and ask for the blessing to make your marriage happy.
The creation of Goddess Kalyanasundari was for the fact that, after the death of Sati, the first wife of Lord Shiva, the world was waiting for the next marriage of Shiva. It is so because, the world was waiting for the son of Shiva and Parvati because according to the boon granted to Tarakasura, he could only die in the hands of the son of Shiva.
So, to give relief to the world, Goddess Kalyanasundari took birth. Eventually, the son of Shiva and Pravati was born and he was named Kartikeya. He killed Tarakasura and saved the world from the problems. The Goddess is regarded as the wish fulfiller for devotees and pilgrims.