me_irl

Gonna expand on that.

It's called the nasal cycle.

And, it isn't necessarily the case that you only breathe through one nostril exclusively. To the contrary, most people will have flow through both. What happens is that the dominance of them cycles.

Back inside the nose, you get boners.

Okay, not really, but kinda. There's these sections that are made of tissue that functions very similar to the way the penis and clitoris do. They fill with blood and swell. Erectile tissue.

Every few hours, one side ~~gets horny~~ swells while the other shrinks. The one that's ~~limp~~ detumesced is where you'll have the most air flow.

However, for most people, that tissue doesn't totally block air flow, it just decreases it significantly. You can test this by blocking each nostril in turn from the outside. Typically, you'll be able to breathe fine from both, but one will just be fine, not fully open and smooth.

While there's no certainty in "why", since evolution isn't prone to developer notes, but the understanding I found back when I first heard about this was that it's a combination of giving each passage time to flush out particulates, remoisturize, and heal any damage taken. Those are certainly benefits of it happening, but we can't really be sure if that's happenstance, or the primary trait that was selected for by environmental pressures.

Test your claim.