Many people buy pillows based purely on looks or price, thinking that "as long as it's fluffy, it's fine." But in reality, a pillow's primary function has never been "cushioning your head," but rather supporting your cervical spine.
In its natural state, the human cervical spine has a slightly forward-convex physiological curve (the neck curve). When we lie down, if the pillow is too high, the cervical spine is over-stretched; if the pillow is too low or empty, the cervical spine is left suspended, causing the muscles to stay tense all night just to protect your spine. Over time, neck and shoulder soreness, stiff necks, and even cervical spondylosis will come knocking.