The purpose of these filters is entirely different. A furnace filter needs to prevent large particles from passing it, as such particles can deposit on the coil

The purpose of these filters is entirely different. A furnace filter needs to prevent large particles from passing it, as such particles can deposit on the coil. Its utility is essentially -(particles missed). A room air purifier exists to remove particles. Its utility is (particles removed). Blowing additional air around that bypass the filter in an air purifier is harmless, whereas it can destroy your furnace or especially your A/C. This is why the IKEA air purifier is a better design than a HEPA purifier. Also people most certainly do build furnaces with gaps around the filter.