A- It helps the ink flow from pen to paper properly. When ink comes out of the plastic tube inside a pen and on to the page, it sucks air with it, creating a vacuum. Outside air will try to flood into the pen to fill this vacuum and if pen has only one hole ( i.e the ballpoint nib) this will make the flow of ink inconsistent as both inrushing air and ink try to squeeze through the same gap. But if the pen has a second hole, the air can flood in through this route without disturbing the ink.