The earth would get warmer and warmer and cooler and cooler as Persephone switched homes. This created seasons on earth that people got used to. When Persephone came home to her mother, it was spring, and when she left, it became fall.
People eventually learned the seasons and knew when to grow vegetables and food and when to prepare for the cold.
However, Demeter couldn't bring warm weather and vegetation to all places on earth. Places very far north and south stayed cold for a long time. So when Persephone came home, Demeter could bring sunshine to only some parts of the world.
Hades loved Persephone a lot and shared his power with her in the underworld. Persephone became the goddess of the underworld and was faithful to Hades.