Rainbows are beautiful. They're also a great way to see the effects of optical phenomena in the atmosphere. Light entering raindrops is refracted, reflected, and refracted again. The red wavelengths are bent less than the blue. If you stand (or sit, or drive) so that the sun is behind you and the raindrops in front of you, and the circumstances are right, the effect is a gorgeous band of colors arcing across the sky.
Rainbows have been important in mythology, art, and culture all over the world, and throughout history. They've been used to symbolize many things, mostly involving a connection between the heavens and earth.
Rainbows are often seen in Hawai'i (that's why they call us "the Rainbow State"). Here, in addition to being beautiful, they actually provide us with some knowledge that can be extremely useful in the short term future. Specifically, if you are driving toward a rainbow, it should remind you to close the moon roof.
If you'll excuse me, I'm going to go finish drying the car.