BJJ rash guards work to prevent a lot of unwanted risks during training such as fingers, hands, and feet getting caught in loose fitting clothes, as well as keeping enough friction and dryness between you and your (sweaty) opponent in No-Gi Jiu-Jitsu. In addition, wearing a rash guard is much more hygienic, creating a barrier between you and unwanted bacteria from your opponent, their clothes, and the mat. Lastly, the name rash guard is there because the material and and long-sleeve element is there to protect you from rashes, or burns from the mat. Anyone who has experienced mat burn will know it is horrible, and can develop into a nasty infection if untreated or poorly maintained.