If you do enough cocaine, you can end up with one nostril. It happened to a British TV celebrity in the 90s.

We have stereo smell to help with locating smells. There's also the nasal cycle. One nostril/sinus handles most of the airflow, then they swap (the sinuses are separate until they get to the throat). That way one can recover moisture, plus some smells are more easily detected with fast airflow and others with slow. So the nostrils functioning differently gives us a broader range of odor detection. What else? Umm, bilateral symmetry and redundancy is useful.

So that we know which side to lay on because one is clogged?

Because we're bifurcated creatures?

Bilateral symmetry, hox genes.