Dreams can be helpful, for example, you're sleeping with your arm under you for well more than an hour and the blood isn't getting to your hand so, your brain makes nightmares and tries to wake you up, or when your heartbeat slows and you get that dream where you feel like you're falling down stairs or off of a platform.
Dreams are strange too, while relying on reality and emotions, some people like my uncle (who was born with a physical disorder) with a hard life almost always have good dreams.
Dreams can sometimes give you déjà vu, it has happened to me before. Something happens in my life and i keep getting the feeling that I've experienced or witnessed this before and later on I remember that I had dreamt that scene before.
Dreams can be supernatural too, I've heard many times of people (mostly relatives) who have seen dead people in their dreams.
Lucid dreaming is another strange thing. If you fail you enter sleep paralysis, which, again, is the body panicking. They can be rewarding if they are successful and fun as you could fly in them (basically controlling your dreams)
Dreams are also closely related to OBE, where one person managed to take two classes at a time while having an OBE, by sleeping in one class while being in the other one at the same time.
In conclusion dreams are strange and not much is known about them. I believe that they have something to do with the soul (if you even believe in stuff like souls).