It's a more complex picture than we first thought. We probably dream all the time we're asleep, just as we think all the time the brain is functional while awake. But some drugs inhibit dreaming, and one man with a brain wound has been shown not to dream at all. Check with your local library. Yes, there are dreams whose explanation is not apparent immediately.
Dreams are very rich in imagery.
Some of the most interesting are those that take many years to fully understand. Every night new data emerges. It's hard to cope with such massive amounts of material, even with computers, although they help. We're not sure how much sleep positions have to do with dreaming. Many people find, however, that they can recall dreams better if they lie in the position in which they originally had the dream.