The BB's fly around your head with 360 FPS and it is almost impossible to prevent that an airsoft BB will hit your face. With a good face mask you know for sure that you will go home without a scratch on your face and with all your teeth intact!
On the field
More and more CQB fields have the rule where they require a face mask to keep the faces and teeth of the players safe. So keep in mind that it is always handy to have one just in case you need it during a CQB event.