What purpose of Demeter and Persephone story?

Ancient Greeks wanted to explain why plants died or lost their leaves in the winter. Demeter was the mother of Persephone. When Hades kidnapped her daughter, she was angry and she let the plants die. Zeus made Hades bring Persephone back. But because she had eaten six seeds, she had to stay with Hades for six months of the year but would live with her mother on the surface of Earth the rest of the year.