Check out this great article from Atos HQ on the importance of training both Gi and No-Gi
You learn how to apply insane pressure (No-Gi)
When you train no-gi you do not have any material to rely on when trying to control your opponent with pressure, as a result you learn what it takes to apply the pressure correctly in order to get into an advantageous position and hold them there. It takes a lot of isometric strength to do this and you will see huge improvements in that type of strength by training no-gi. No-gi accentuates any time that you are too loose with your positions or you get sloppy so you can correct these mistakes in your techniques. All of this will translate perfectly into making your gi game stronger, and your ability to control your opponent will improve immensely.
You’ll develop quick reflexes (No-Gi)
No-gi is a much faster pace fight because you do not have the ability to grip your opponent’s gi and tie them up to slow them down. You have no choice but to act quickly and instinctively or opportunities will pass you by, and your training partner will get a step ahead of you. Movements you’ve practiced will become second nature and you’ll develop quick reflexes, which will directly translate into your gi game.
You’ll develop strong grips (Gi)
It is essential for any Brazilian Jiu Jitsu athlete to have extremely strong grips, by training gi you develop these unbreakable grips. The constant grabbing and pulling of the gi will help your grip strength and endurance, which will add to the power and strength of your no gi grips as well. Even though the grips in no-gi are different and you’re grabbing the wrist instead of the sleeve it’s still directly improves the development of your grip strength.
You’ll have excellent escapes (Gi)
None of us want to ever get in a bad position but it happens and when it does you have to know how to escape. The friction of the gi makes it much more difficult to escape a bad position which forces you to know the correct technique. When you are confident with your escapes you’ll be more willing to attack because you know how to get back into a good position. If your escapes are sharp in gi you’ll have no problem in no-gi.
You’ll have a deeper knowledge of BJJ techniques (Gi)
Gi is much more technical then no-gi because of all the grip options and complex guards that have evolved over the years. This in turn forces you to develop a more technical game so that you can stay one step ahead of your opponent. It is important to develop this base Jiu Jitsu because it will make the transition to no-gi much easier and put you ahead of the game.
You’ll be well prepared in any self defense scenario
If you get into a sticky situation in the real world and you need to put your Brazilian Jiu Jitsu skills to the test you will be fully prepared for any self defense scenario if you have been training both gi and no-gi. If they are wearing a jacket you have your gi grips and you can calmly handle the situation. When the grips aren’t there you still know what to do and you can easily be explosive and act on instinct thanks to your no-gi training.
You’ll have the most well rounded take down game
A lot of Brazilian Jiu Jitsu practitioners like to pull guard and there’s nothing wrong with that, but it is important to be well rounded and know take downs too. In gi judo style take downs are more popular and in no-gi it’s usually all wrestling style take downs. However, both styles transition well into either gi or no-gi and it’s important to be familiar with both. Brazilian Jiu Jitsu matches always start from the feet so do yourself a favor and get comfortable on your feet so you can be prepared for anything.