I am an amateur boxer competing so perhaps I can share some insight.

Basically breathing technique is a major aspect one needs to learn. You can very quickly gas out(lose your breath, which also leads to a lack of oxygen for your essential muscles). Therefore, we learn to breathe through out noses when you're not tense - i.e. when you jumprope and such.

However, the inflow and outflow of air is very different when punching and receiving punches.

First of all, you need to exhale fast when you punch, usually done through your mouth(rather than nose), and through your teeth(you don't want to open your mouth, your jaw can break if you receive a punch). Basically you do it fast when you punch and inhale slowly with your nose afterwards because you don't want a stomach full of air when you get potentially punched in it - I'm sure most people have tried getting the air knocked out of them, and it's terrible and could lose you a boxing fight!

Boxing is about repetition and muscle memory so you do it when doing bagwork and mittwork as well, to make it second nature.

TL;DR: It's difficult to be punched and you don't want it to happen with a stomach full of air.

I am an amateur boxer competing so perhaps I can share some insight.

Basically breathing technique is a major aspect one needs to learn. You can very quickly gas out  (lose your breath, which also leads to a lack of oxygen for your essential muscles). Therefore, we learn to breathe through our noses when you're not tense - i.e. when you jump-rope and such.

However, the inflow and outflow of air is very different when punching and receiving punches.

First of all, you need to exhale fast when you punch, usually done through your mouth  (rather than nose), and through your teeth  (you don't want to open your mouth, your jaw can break if you receive a punch). Basically you do it fast when you punch and inhale slowly with your nose afterwards because you don't want a lung-full of air when you get potentially punched in the stomach - I'm sure most people have tried getting the air knocked out of them, and it's terrible and could lose you a boxing fight!

Boxing is about repetition and muscle memory so you do it when doing bag-work and mitt-work as well, to make it second nature.

TL;DR: It's difficult to be punched and you don't want it to happen with a lung full of air.