We sometimes assume that, just like everything else, our headlights are all about us. They help us see what's in front of us, kind of like a pair of big flashlights that don't need their batteries replaced every time your power goes out. Actually, when it's raining, what you need your headlights for, even in the daytime, is so that other people can see you. In the murky gloom of a rainstorm, even a light drizzly one, the lack of sunlight makes the world look like one big mass of overcooked oatmeal and seeing a pair of headlights coming out of that oatmeal will remind other drivers that -- whoops! -- there's something in that oatmeal besides raisins!
But if headlights help you be seen, you don't want to unintentionally blind people with them either. Other drivers are already having enough trouble finding their way around. Don't dazzle them by turning on your high beams.