While the obvious answer to using a mouth guard is that it protects your teeth from fractures and from being knocked out. It also has other purposes.

The main purpose of a mouth guard is clear. It is there to protect your teeth from possible direct blows where they can either be chipped, significantly fractured, or knocked out. While serving a purpose in this regard, they also help to prevent some of the "through and through" lip and cheek lacerations which can happen when a tooth is broken.

The other purpose of the mouth guard is to basically act as a shock absorber in your mouth. It serves as a spacer between the top and bottom row of your teeth and absorbs shock should you receive a blow to your head or jaw. You can image that if you have a significant blow to your chin that this force is going to go from your chin when you are checked up through your jaw bone, into your teeth, into your facial bone, and then into your brain. In this regard, we believe that the use of a mouth guard helps to decrease jaw bone (mandible) fractures. In addition, it is also believed that the use of a good mouth guard helps to decrease the chance of concussions. It does this by helping to absorb the force that your jaw may pick up when there is a significant blow delivered to it. Since a concussion is basically a big bruise to your brain, which could result in permanent brain damage, it is important to try to minimize your exposure to things which can cause concussions. In this regard, it is highly recommended that you wear a mouth guard when you expect to play contact ice hockey. Obviously, other than skating by yourself, almost everything in ice hockey is contact related.

I hope this answers your question and that you choose to wear a mouth guard in all ice hockey related activities in the future. Wearing a mouth guard, and the rules which enforce it, are sort of like the rules for having seatbelts in cars. The mouth guard serves as a seatbelt to protect your teeth, jaw bone, and brain.