Wearing a rash guard while competing in no gi jiu jitsu is a requirement. The main purpose of rash guards is to protect your body from picking up bruises and scratches.
If you are practicing or competing for a no gi BJJ competition then wearing a rash guard is a compulsion. You also need to wear a rash guard underneath your gi according to the IBJJF guidelines.
If you wish to add an extra layer of protection against bacteria and germs then you must have these rash guards.
There are several benefits of wearing compression spats in BJJ that include improved blood flow, minimum muscle vibration to reduce the risk of injury, and many more.