Why does BJJ self-defense require Gi or No-Gi training?

If you are a beginner in Brazilian jiu-jitsu, one thing comes across your mind, is it better to train in Gi or No-gi? But, let me tell you both of them have their own advantages, and it all depends, whether you feel comfortable training in Gi or in No-Gi. It has been observed that GI provides you with the proper defensive fight, and also helps to absorb your sweat.