We dream to wake up to our larger Self: to the memory of who we are and what we are meant to become.

There is a wonderful Dine (Navajo) teaching story about this.  In the Fourth World, the Hero Twins traveled the earth slaying monsters.  They killed Big Giant, and Horned Monster, and the demons of old age and cold and poverty.

When they attacked the demon of Fatigue, he begged them, “Don’t kill me!  If you kill me, people won’t get tired.  And if they don’t get tired, they won’t sleep.  And if they don’t sleep, they won’t dream.  And if they don’t dream, how will they stay in touch with the spiritual world?”

So the Hero Twins let Fatigue live — so people would sleep, and dream, and return in dreams to the deeper dimensions of reality where the events of our waking lives have their source.

This simple story contains a better answer to the question, “Why do we dream?” than any speculation you are likely to read about brain processing.   Dreams are both the language of the soul and experiences of the soul.  If we did not dream, we would be soul-gone, something less than human.