Why the coffee package will have the coffee valve on it?

First,the coffee after roasting ,will release more and more co2,while you pack into the coffee packaage which without the coffee valve in it, the co2 will full of your package that will make your coffee bag brust.
Second,if the co2 full of your coffee package,and the co2 full of your package,it also will oxidate your coffee bean or coffee powder.

That is why the coffee package will have the coffee valve in it.