What would happen if your nostrils were facing up instead of down? Rain would fall into your nose and when you were sad or with a cold, mucus would fill up in the nose instead of draining, creating quite a problem. Then why do we have two nostrils? The reason being, if one is blocked, we can breathe through the other (unless you have bad hay fever or a cold and both are blocked). This shows how (almost) every part of the human body has a purpose, including its shape and characteristics.
Another fun fact about nostrils is that at one moment, only one nostril is used for breathing. In other words, you can breathe easily through one side of your nose but the other side will feel stuffy and blocked. This phenomenon alternates sides on a periodic cycle (where the blocked side becomes clear and vice versa). This mechanism is most likely to protect the inside of your nose (nasal cavity) from drying out.