system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do humans like pillows to support their heads when they sleep?

Other animals don't need any sort of head support, and if museums are correct, humans have sought out sleep-time head-supporting objects for millennia.
gpt
The spine of primates is curved to support standing upright, so it does not do as well lying flat on our back or stomach.  We can do so, but it is not as comfortable as lying on our sides.  

Primate shoulders stick out past our head, so when we lay on our sides our heads are not lying flat on the ground (unlike dogs or cats for instance).  The larger the primate the bigger the gap (monkeys have tiny shoulders and are very flexible so they can usually just shift their shoulder out of the way).  Therefore to support our heads we need something to  fill the gap between our shoulder and head.

Google "ape sleeping" and look at the pictures.  You will see that apes also sleep on their sides with their arm under their head.