What is the reason why bees see ultraviolet light?

That's the reason why petals are usually a different color than leaves. Even though humans can see more colors, bees have a much broader range of color vision. Their ability to see ultraviolet light gives them an advantage when seeking nectar. Many patterns on flowers are invisible to humans. read more

Bees evolved from wasps, which can see in UV (as can many insects, so the ability to see in UV likely happened much earlier in insect history). When the partnership between bees and flowers happened, many flowers developed patterns which can only be seen in UV. These act as a guide for bees and other pollinators. read more