Flowers emit fragrance primarily to attract insects. This is because insects play a crucial role in the pollination process, where they transfer pollen from the male reproductive parts to the female reproductive parts of a flower, enabling the plant to reproduce. The fragrance acts as a signal to attract insects towards the flower.

Option 1, "purify air," is not the primary reason for the emission of fragrance by flowers. While flowers do release oxygen during photosynthesis which can contribute to air purification, the main purpose of fragrance is not air purification.

Option 2, "drive away flies," is also incorrect. Flowers do not emit fragrance to repel flies or any other pests.

Therefore, option 3, "attract insects," is the most accurate explanation for why flowers emit fragrance.