When sleeping, pillows can help you keep the curvature of your cervical spine.
It doesn't matter if you don't sleep on the pillow. Maybe you are young, or you are used to it. A newborn baby can sleep without a pillow. When our spine is formed, pillows protect our shoulders, neck and spine. Our shoulders and necks are bent.
The function of pillows is not only for us to use, but also to protect our shoulders and necks.
I hope the following photos are helpful.