Depends on your setup and where you keep the Antminer(s).
The main reasons for me are heat extraction and noise reduction because I live in a one-room apartment running several Antminers in soundproof boxes.
If you attach a hose to Z9 mini without an extra exhaust fan, the intake fan would have to run at max fan speed most of the time to be able to keep the Temp(Chip) under 85.
Ahh, I always forget people try and run industrial mining equipment in places where they really shouldnt lol
Hehehe just a man on the street trying to make a living.
The upside is that they run no more noisier than GPUs now, I can actually sleep without earplugs.