What does the myth of Persephone explain?

Persephone is the only daughter of Demeter (Ceres) the Goddess of the Harvest and Zeus. Hades, god of the underworld, loved Persephone's beauty and kidnapped her and brought her to the underworld. When Zeus had discovered what Hades had done, he sent Hermes to the underworld demanding Persephone were released. Hades obliged but insisted the Persephone eat a pomegranate seed before he left. Thinking nothing of it, she agreed not knowing it would force her to stay in the underworld. Zeus and Demeter were furious so they struck a deal with Hades letting her live half the year above the surface ans spend half of it below, with Hades. Whenever Persephone has to retreat back to the underworld, Demeter gets horribly depressed and refuses to let anything grow, hence fall and winter, and when she is back, Demeter is happy and spring and summer flourish.