It's a very interesting question. I think there are a number of reasons for this. The first is about information. Information is a key part of human survival, if one travelling group of nomads meets up with another they will no doubt want to share their adventures, perhaps give tidings of good or bad news. Perhaps one tribe might also want their story to sound more fascinating than it actually was, so imagination becomes important. Perhaps they may want to deceive the other tribe, in which case the ability to create fictional realities also becomes important. It might also be important in fulfilling the urge to know everything, hence the beginnings of religious thought.

This explains fiction in a crude way. I'm sure such a topic deserves further study and I'm sure there probably are some studies there.

Creativity in general serves another purpose. Humans are very intuitive tool users. From spears to eating bowls to sub-machine guns. The need to create tools is important if we are to survive in not only defending our relatively weak bodies and in our gathering of food. The desire to produce becomes engrained in our psychology. Produce or die has that effect, those that were naturally producers survived, that creative need is part of all of us. Whether it be fine art or plumbing.