Rash Guards are a essential for no-gi BJJ and MMA as they wick away moisture and limit skin-on-skin contact. This will not only keep you more dry, it will also help prevent contraction of skin born diseases like ring worm. This makes rash guards popular for training and competition. Further, they are necessary for no-gi IBJJF competition. Rash Guards are also great for use under your gi as they will still limit skin-on-skin contact and they also minimize the rug-burn like effect that the gi can have on your body, especially your elbows.