If you imagine that a fully filled car tire has around 2-3 bar. So, if you would put a hose in it with a mouth piece and try to breathe from it, it most likely wouldn’t work as the pressure is too high to compensate. Same thing in Scuba Diving with your tank filled to 200 bar this would never work. That’s why we have a SCUBA (Self-Contained-Underwater-Breathing-Aparatus) Regulator.
The function of it is quite simple;