What is the function of a pressure reducer valve in a oxygen system?

Oxygen generally comes compressed in a cylinder or a bank of cylinders...Some places like hospitals even have cryogenic receptacle oxygen(liquid stuff)...the pressure we are looking at here is much higher than atmospheric lets say 200 times more so to get it back to a more usable pressure at a rate we can use a pressure regulator is used..In some applications more than one in series with another is used as the use of one creates such a drop in pressure that freezing is a big concern...After all if we had our oxygen mask hooked up to about 50psi it would blow your lungs out the other side...well that's an ugly picture....:) Have a great day remember one atmos = 14.7psi at sea level..+/- a bit....Good luck from the E...