So Under Armour, and any dentist willing to pay money to advertise with them, has been marketing a mouthguard that will improve athletic performance. You really have to be a dumb jock to believe that, but for those that were tempted, research has in fact proven there is no difference between their mouthguard and others. In fact a store bought one scored lower than a custom no name brand mouthguard in terms of comfort, no difference in terms of performance.
This should not be a surprise to anyone who is not a sucker, but companies are good at turning us into suckers when we want something, ie better performance. Here is the mouth guard my wife and I recommend for ortho patients. I do not think it matters what you use if you do not have ortho, but a custom one is more comfortable and less likely to fall out constantly. That being said it is far from necessary.