Apparently breathing through your mouth damages your teeth, prevents you from absorbing as much oxygen and doesn't clean, condition and warm the air the way breathing through your nose does. It can also alter the structure of your face.
nose hairs filter out dirt and grime. all the dirt and grime fucks up your teeth and goes straight into your lungs when you mouth breathe. I think everyone does both, but tend to do nose breaking more often.
I have a bit of a problem with these kind of suggestions around breathing. The idea that there's one simple right prescription seems false to me. Breathing differently can have huge, radical effects but people extrapolate from this and assume there's one right way, and I don't think that's true at all. I played around with loads of yoga (pranayama) rhythms when younger and it's definitely fun and interesting, but in general the most radical stuff happens when you let the body be, and let it find it's own rhythms. I would say same about mouth/nose.