A pillow insert is not a complete pillow, but rather one of its main components.
A complete pillow typically consists of two parts: the pillow insert and the pillowcase. The pillow insert is the internal filling, responsible for providing support, height, and comfort. Common filling materials include buckwheat hulls, down, latex, memory foam, and cassia seeds. The pillowcase is a removable and washable fabric cover that wraps around the pillow insert, primarily serving to protect it, keep it clean, and enhance its feel and appearance.
In short:
Pillow insert = Support structure + Filling material
Pillowcase = Outer protection + Easy cleaning
Pillow = Pillow insert + Pillowcase
Therefore, although people often use "pillow insert" and "pillow" interchangeably in daily life, functionally and structurally, the pillow insert is only the core part of a pillow and is not equivalent to a complete pillow.