Why do we have two nostrils?

The two sides of the human nasal cavity (called nasal passages) are not redundant. At any given time, the flow of air through them is different, and the higher air flow periodically moves from nostril to nostril.

A possible evolutionary explanation is that some odors are better detected in relatively fast air flows while others in slower ones. Our brain also perceives and processes a wider range of odors by comparing the signals of the olfactory neurons on both sides.

Our sense of smell could, therefore, be poorer if we had an undivided nose and only one airflow.