It’s more like one nostril is high-velocity and the other is low-velocity, not that one is completely blocked. The evolutionary reason is thought to be that it aids the perception of low PPM volatile organic compounds (i.e. bad smells = rot, toxic chemicals) because the ‘blocked’ nostril is used as a sense chamber which lets the air spend more time within, thus increase the effective sensitivity of your sense of smell.
If you do not have this behaviour (which can disappear with certain neurological diseases or just idiopathically not exist) you also likely have a reduced sense of smell.
Can confirm that I "smell" through one nostril at a time. I can smell more sensitively than others (for example, realise that milk is rotten before others do). I have noticed that when I try blocking one of my nostrils, my sense of smell is considerably lower, and it is not always the left nostril or always the right nostril. It kind of alternates, and I don't know the pattern. It might be that both nostrils alternate doing the task of smelling at different times.
I'm probably one of them then, because I've tested this on and off for years whenever someone makes the claim, and it's never true for me.
It's just really annoying how many people say with absolute certainty that everyone does it.