Asking for the roots of dreaming seems like asking „why do we eat“ or „why do we need friends“ – we’re meant to live this way. In my opinion, the real question is: Is a dream significant, is it meaning something?
I would say, in this case the radical constructivist approach makes sense: you create its meaning by believing in it. If you believe, dreaming is nonsense, then you will rarely remember „special dreams“. If you think, dreaming is processing reality on another (subconscious) level, then you actively allow this processing to happen. And if you think, dreams are forecasting reality in some way, you will look back and actually find similarities.
Maybe the difference between dreams and day dreams is not as big as we may think. Both refer to our innermost values and attitudes, so deep inside we often don’t even realize them. And both can cause humans to break out of their tracks, beginning to write a new history.
If you really want my personal opinion on that: dreaming is like living in another world, with different laws of gravity. And sometimes it’s scary to see how much and how little at the same time is depending on my personal will in this world. Sometimes I feel like a supreme king, sometimes like a chased cat. But most of the time, I feel like a child receiving a present and playing with it freely. It’s like eating: without it, I would gradually starve.
So do dreams have meaning? They can. Are they reflections of day experiences? Sometimes. And sometimes, even God can speak through them, bringing new ideas and behaviours.