Your nose and sinuses make 200-300cc of fluid a day. Most of the time, it is transported to the back of your nose, and swallowed down your throat with no problem and you don't notice.
When it becomes infected or too thick, it will hang up and"stick more" and irritate the linings of your throat and well as have little nasties growing in in.
Also, as it drains down, it can pool in the larynx, just around your vocal cords and it is there where it is supposed to be swallowed, down the esophagus, but can hang up and make you cough. It can also find its way down into the trachea, making you cough as well. It can be aspirated into the lungs as well. This is not a problem for most people, but there is a link between chronic sinus disease, allergic rhinitis, and asthma.
Through a little tobacco smoke in for good measure and watch some folks cough up a lung!
Nasal saline irrigation and Mucinex are your friend.
You make think it's funny, but it snot.