It is so the paper media isn't perforated. I blow across the media rather than directly through. For OPE, some manufacture's are for blowing them out, some don't.I've blown out the paper element for years and years on all my serviced mowers. Without issue I might add. Does anyone know the reason why they don't recommend blowing out the air filters ? Like mentioned personally I've not had any issues with this method. Even on my Ridgeline I've done this and had an oil analysis done afterwards and it was even mentioned that the silica in the oil was low and the air filter was working well. Anyone know ?