Why I run an expensive HEPA air filtration system 24/7 even though the air is relatively clean. Although I live near a busy highway, the air is effectively 0 pp

Why I run an expensive HEPA air filtration system 24/7 even though the air is relatively clean. Although I live near a busy highway, the air is effectively 0 ppm (undetectable) in each of 1.0, 2.5, and 10 um at home indoors. (BlueAir 680i with the better 2-part filters.)