There is a pretty good chance that you will have used a BIC pen at some point in your life, and if you have one nearby just now, then you might want to pick it up and check it out. If you look closely then you will see that there is a small hole in the side of the pen, and why do you think they have included a hole in the design of the pen?
It turns out that the hole, and there are actually two of them, need to be in the pen as they allow the pressure to be the same both inside and outside of the pen which then, in turn, basically means that it is then able to work. If the pressure was different, then the ink would find it harder to flow to the nib of the pen and you would then be cursing that your BIC pen had all dried up even though it clearly had ink left in it.
So, a simple explanation for such a small thing and yet it plays such an important role that it should never be overlooked at any point.