Why do we dream?

I just watched the movie Inside Out for the one millionth time and while I was watching I started wondering why we actually dream? In the movie they say its just for entertainment to the tiny people living inside you but I what to figure out the real reason.

Here's what I got:

After doing some research I came to conclusion that there is no scientific reason that we dream yet however any theories say that dreams are a way to process all the events and emotions we experience throughout the day. Which if you think about it it's kind of true.