Right after roasting whole bean coffee gives off co2 gas. Without the valve fresh roasted coffee could cause a sealed bag to expand to the point of bursting. So the valve lets the co2 escape.
The degassing valve helps the coffee package maintain its shape by allowing gases to escape. Gases are emitted after roasting and continue to emit even after the coffee is packaged. If the gases are trapped inside the package, this can cause the bag or pouch to inflate or bloat.
