My theory on why we dream is a bit of a combination of psychoanalytic theory and the idea that our brains use our dreams to "clean up" the unfinished business of the day. It's hard to imagine from the content of some of my dreams, but I imagine that I dream as a way of processing things from my day that I couldn't make conscious sense of. I reckon our dreams also drag in lots of our other unconscious desires and fears. I don't subscribe to a substantial chunk of psychoanalytic theory, but my guess is that Freud may have been on the right track here. If my dreams really are "disguised fulfillments of repressed wishes," I must secretly wish that I lived in a world where physics doesn't (don't?) work right. The psychoanalytic guess is at least more satisfying than thinking that my brain is just trying to make sense of random signals being fired off by my limbic system.
I don't usually try to interpret my dreams. They fade away so quickly I usually can't even remember enough of them to try.
I don't rule out dreams as a way for God to reveal Himself today, but I would be reluctant to trust anything in my dreams. When I read stories about Biblical characters like Joseph and Daniel, I'm amazed at the clarity of peoples' dreams and at how definite the interpretations are.