The ones you sleep on, that is. My parents failed to equip me with this information growing up, and I’ve changed cities or countries often enough for it not to have been an issue up to now. I don’t know about you, but my pillows get kind of yellow after a while. Plus, they lose a lot of their pillowy softness.
Additionally, there’s the multi-pillow factor to consider. If you, as I sometimes do, sleep with two pillows (neither of which is a body pillow, thank you) presumably that doubles the life of each pillow. Assuming you don’t distinguish between them. If you have a “top” pillow and “bottom” pillow in the stack (keep your gay slang jokes to yourself, thank you), then probably their longevity is affected unevenly. Someone needs to do a PhD on this.