I got my landlord to install one, there was already a hole above the stove that was boarded up so I think it vents somewhere (hopefully outside). When I moved in there was a layer of grease over the whole kitchen so I thought maybe a vent would help (and obviously the previous tenant could have cleaned, but apparently didn't know what that was). Maybe it wasn't mandatory to install it, but I really like having it and I think every kitchen should have one if it isn't too difficult/expensive to set it up.