Why do we dream during sleep?

Science has made great progress in deepening our understanding of dreams. Still, there is no answer to the question:

Why do we dream during sleep?

I thought a lot about this question and whenever i read i didn't get anything!
Is the dream a real thing we live?! or just a wasting time during sleep. If we all thought about dreams, we will get different opinions. However, the most important part of this topic is "What is the THING that causes dreams?! "

I believe that everything in this life has a positive impact on us, but i never knew the benefits of dreams! let's assume that the dreams have benefits, so why we dream it during sleeping only! Can we consider it as an organic DMT pill ?! I think the people who are trying DMT to reach the peak of Happiness and live situations that don't exist, they can reach this point during sleeping, so it's the same idea.

Did anyone tried to link imagination of dreams with the reality of life ?.. so if the LIFE is a DREAM, So we are living in a story and is gonna end when we die. Also one of the most questions that in my mind is "Why am i dreaming this specific dream in this day?! ".People say that dreams are based on our subconscious, Ok! so when we are dreaming 'Imaginary Dreams', what were we thinking about ?!... For me, the relation between subconscious and dreams is not strong enough and we can't consider it the main cause. Most of the kids think that there is someone choosing their dreams every day! Maybe because the adults escape answering this question. Anyway, the topic has lots of questions that i didn't get any answer.

I won't give up to search for an answer to satisfy me so please, Share your knowledge.