I have allergies. I remember walking home from work one day thinking "Ah, my sinuses have cleared, the pollen must have finally gone." only to wake up the next morning all stuffed up again. Then it dawned upon me that work has HEPA filtered air, but my bedroom does not despite spending ~⅓ of my life there. Having an air purifier in my bedroom means I've avoided almost all symptoms of allergies for the last 5+ years without needing to resort to medication.