1. When you roll with a gi the grips only work from topside if you put pressure in the right area. Once you have experience in the gi, your ability to put pressure on the right areas to maintain positions will be much higher. Areas like the diaghram, femur, jaw, and other not so obvious spots.
2. When you roll with a gi their are grips you can get on certain parts of the limbs from different positions and scenarios that kill your opponents movements. This works in no gi too, but it is very difficult to practice because the game moves much faster. After practicing in a gi your hands will go to those spots more naturally than a no gi only player which will help you kill guard work faster.
3. When you roll no gi you can slip out of submissions more easily, but with a gi you have to know exactly where to push and HOW to pull. After doing gi work you will instictively know how to break submissions at different stages of their implementation but will now be able to apply the slipperiness and explosions that nogi provides.
4. As mentioned before, in gi work if you mess up your posture you will get killed. Defending gi chokes inside your opponents guard is all about protecting the biggest lever in your body, the spine. After tons of practice in the gi you will retain good spine structure naturally which gives you a much stronger base, which will help you defend just about any rubber guard attack even if you have never seen the set up before.
5. In gi work you have more options for open guard which is all about shrimping, framing, and most importantly knee rotation. While spider guard isnt very useful in no gi, a good spider guard player will have enough leg and shin dexterity to defend many guard passes naturally in no gi. Especially if they are based on speed and not pressure.
I hope this gives you some ideas. IMO it is possible to get really good at BJJ without touching the gi, but it takes a teacher with a lot more vigilance, discipline, and knowledge. The gi is darwinism at it's finest. If your fundamentals are wrong while wearing a gi you will get tapped until you fix the problem. That rule doesnt apply as often without the gi. You don't need as much careful coaching without the gi, because gi training regulates itself. That's why it is considered more techincal.