the importance of a mouthpiece

I can't stress enough how important it is to wear a mouthpiece while you're lifting. The loads you're forcing your body to tolerate  while strength training almost require you to clinch your teeth.
This can be a real detriment to  your tooth enamel. With the constant clenching of the teeth a mouthpiece will protect your teeth and I vaguely remember from somewhere on the internet that clenching your teeth could lead to gaining strength.  Check it out here.