The main reason why athletes choose a custom-fit mouthguard when they play a contact or combat sport is to keep themselves safer than a boil-and-bite mouthpiece would.

There have been countless examples of teeth being saved by a good mouthguard and, conversely, fights lost by an ill-fitting piece. World Rugby even mandates smart mouthguards to help protect against the long-term effects of concussions and head injuries.

However, there are also studies which suggest that the right mouthguard not only protects an athlete and ensures that they can continue competing without being substituted or having a doctor stop the contest, but also that it can improve athletic performance.

Part of this is related to protection; a significant aspect of high-level competition is psychological in nature, and being certain that you can perform at your best without getting injured allows you to be less apprehensive and focus on the task at hand. 

Somewhat paradoxically, being less afraid of injury can often lead to a lower chance of injury, as tensing up can increase the risk of some injuries (albeit lowering the risk of others).

Beyond this, a proper mouthguard shifts the alignment of your jaw, which is believed to help with muscular strength due to the position of the jaw when it is tensed up. Whilst this remains a preliminary field of study, it is also the principle for which mouthguards are believed to help reduce the risk of concussion.

As well as this, a properly fitted mouthguard allows for the airway to be opened efficiently, which allows for easier intakes of breath, which in turn helps with stamina, endurance and sporting performance in general.

Most athletes are as good as their cardio allows them to be, and the right mouthguard ensures they can be as effective as possible. Some mouthpieces fit so well that people can talk whilst using them, a vital benefit in team sports such as basketball and rugby.