A properly adjusted mask is the difference between a headache and a funny story.
Obviously a mask that is too loose will interfere with your ability to call a game. A loose mask can move and obstruct your vision. It may also cause you to tighten your jaw to stabilize the mask and therefore subject you to possible injury when the mask is hit. It could also shift when your head moves causing your head or face to become exposed.
A mask that is too tight can cause a concussion when the ball hits the mask. The energy of the blow is absorbed and transferred to your head instead of the mask. If your mask doesn't move when hit, it is on too tight.
The proper fitting mask is somewhere in between. It is easy to put on and take off with one hand. It also will move (or spin) when hit, thereby distributing the energy of the blow.
A mask will also loosen during the course of a season, so check your equipment regularly and make the proper adjustments.