Answer:
To explain seasons.
Step-by-step explanation:
If you're talking about the story where Persephone stays with Demeter for half a year, and Hades with the other half, its to explain seasons.
The concept was that when Demeter and Persephone were together, they were happy, warm, and sunny; like spring and summer.
When Hades and Demeter were together, it was cold, chilly, lonely, and dark, like the fall and spring.
Thus explaining the seasons.