I think there are a ton of reasons, including some already stated. One I have not seen mentioned is that frankly a lot of "modern jiu jitsu" movements and concepts are better conveyed in the gi. For example the entire Atos/Mendes Bros style of guard passing and distance management guard play is just more easily taught conceptually in the gi. Yes, you can do berimbolos and kiss of the dragons without the gi. You can do them very well. But it's easier to learn them in the gi and then move those skills over. The same goes with some of the leg drag pass variations, and shin slicing to pass the spiral guard. Learning spiral guard or Delariva is just simply easier in the gi. There are some other examples but it's just running it into the ground.
Now this may seem silly at first. Can't you just learn them and perform them fine without the gi? Of course! I just don't think it's as efficient. It's a logical fallacy to appeal to authority, but again, everyone wants to look at Marcelo Garcia for advice. What did he say about the north south choke before he felt comfortable submitting back to back opponents with it at the 2010 Worlds? he said he honed the position in no-gi first. It makes perfect sense. Same with his guillotine. It's much easier to land darces, guillotines, north south chokes, anacondas, and arm triangles in the gi if you've gotten experience with them in no-gi first. The same goes with the berimbolos, your kiss of the dragons, kiss of the tigers, leg drags, squeeze drags, shin slices, whatever, but they translate better from to no-g if you learn them in gi firsti.
I don't care how much you hate or love the gi. Learning the berimbolo while being able to grab the belt and the pants is very important, and it WILL help you do the berimbolo better without the gi. But if you don't feel like doing it in the gi, don't do it. That's great also. I enjoy the gi but I have a very Marcelo Garcia or Jeff Glover type approach where I don't use a lot of collar chokes or other grip reliant techniques like the spider guard or collar chokes.
I also think no-gi helps gi, which wasn't the question asked. But I think that the transitional attacks, the pace, and the level of which your finishes have to be dialed in laser sharp definitely helps your offense when you go back to the gi. I think they both are two sides of a coin, and they help each other. Last thing I'll say on this, and it applies to people that make arguments for or against gi/no-gi training or whatever agenda they have (if they have one.) When people list a bunch of names of successful gi guys that won ADCC, that doesn't prove anything. Same thing with listing a bunch of guys that never wore the gi that won ADCC. There are so many factors involved that go beyond just wins and losses.
Marcelo Garcia is someone I just did that with in this post. He's the best no-gi grappler ever, so of course his opinion has a lot of worth. But he also has opinions that other grapplers that are equally skilled disagree completely with. For example, Marcelo doesn't like kimuras or arm triangles. He loves guillotines and NS chokes. Rafa Mendes is the opposite and thinks the guillotine and north south choke are more risky and harder to finish, and prefers arm triangles like the anaconda or darce. Then other legends like Cobrinha, Rafael Lovato Jr, and Jeff Glover think they're all great. It's important to be careful of hero worship. Everyone brings their own biases to the mat, including our favorites. Who is right? Marceo? Rafa? Both? Neither? You have to do what works for you, and find your own path, which brings me to my last point.
I also have all the respect in the world for Eddie Bravo and 10th Planet, and I think you gotta do what makes you happy. Right now I do about a 50/50 no-gi to gi split. I wish I could do 2/3rds no-gi, but my gym just doesn't offer enough classes. I wish I had a 10th Planet school close by. You gotta do what makes you happy in life, and no one should train anything that doesn't make them happy or benefit them. So I just made a fairly pro gi post, but keep in mind that I'm not preaching at all, and I'd train way more no-gi if I could at my academy.